since the 
 discovery of a three - coordinate terminal nickel imido , ( dtbpe)ni = nar 
 ( dtbpe = bu2pch2ch2pbu2 , ar = 
 2,6-pr2c6h3 , scheme 1a ) in 2001 , this class of complexes remains relatively scarce , and 
 their reactivity remains underexplored .
the low coordination geometry offered by the strong -donating 
 and sterically encumbered chelating bisphosphine ligand allows for 
 optimal  and  bond formation in a trigonal planar environment 
 without causing  electron  electron repulsion between 
 the electron - rich late transition metal and the -loaded imido 
 ligand .
three - coordinate nickel(ii ) imidos 
 can also be prepared with other sterically demanding groups on the 
 imido nitrogen , such as mesityl , 1-adamantyl 
 ( ad ) , and dmp ( dmp = 2,6-dimesitylphenyl , scheme 1a ) .
the 
 use of a bulky scaffold such as dmp allows the isolation of a three - coordinate 
 ni(iii ) imido by virtue of a one - electron oxidation process ( scheme 1b ) .
warren and co - workers 
 have also found that -diketiminate ligands can stabilize ni(iii ) 
 complexes possessing the terminal imido ligand ( scheme 1c ) , while limberg has recently 
 expanded this approach to a diazenido ligand using a more sterically 
 encumbered chelating ligand ( scheme 1d ) .
other examples of kinetically stabilized ni(ii ) 
 imidos have been reported using a sterically encumbering and chelating 
 bis - n - heterocyclic carbene ligand ( 3,3-methylenebis(1-tert - butyl-4,5-dimethylimidazoliylidene , scheme 1e ) .
hillhouse 
 reported examples of linear , two - coordinate imidos using an impressively 
 sterically demanding n - heterocyclic carbene as well as a bulky dmp 
 group on the imido nitrogen ( scheme 1f ) . apart from their fascinating 
 coordination and electronic properties , terminal imido complexes of nickel are also highly reactive and 
 effect h atom - abstraction ,
engage in 
 c  h bond activation chemistry such as ethylene amination , the amination of allylic groups such as 1,4-cyclohexadiene , 
 and even activation of stronger c  h bonds in substrates such 
 as indane , ethylbenzene , and toluene .
h bond activation proceeds via 
 hydrogen atom abstraction and alkyl radical formation , followed by 
 recombination of the latter with a second equivalent of ni - imido to 
 form primary and secondary nickel(ii ) amides .
the radical nature of some ni(iii ) imidos is also manifested in 
 radical recombination to form diphenoquinonediimine - type scaffolds .
lastly , nickel complexes with a terminal imido ligand 
 can also engage in nitrene group transfer to unsaturated molecules 
 such as alkenes , organic azides , co , and cnr .
surprisingly , the reactivity of nickel imidos 
 toward other unsaturated substrates including cumulenes , has not been 
 reported despite the rich chemistry from the similar species ( dtbpe)ni = cph2 and ( dtbpe)ni = p[dmp ] . in this study , we describe 
 reaction chemistry of the first nickel(ii ) complexes having a terminally 
 bound imido ligand , ( dtbpe)ni = nr ( r = 2,6-pr2c6h3 , 2,3,6-me3c6h2 ( mes ) , and ad ) with various small unsaturated 
 molecules .
these ni(ii ) imidos can abstract an h atom from a reductant 
 such as hsnbu3 , but are also basic enough to heterolytically 
 activate the c  h bond of a terminal alkyne
. other substrates , 
 such as carbon dioxide , isocyanate , aldehyde , and ketene functional 
 groups , engage in [ 2 + 2 ] cycloaddition , and sometimes insertion chemistry , 
 across the ni = n bond to form stable and crystalline 4- and 
 6-membered ring nickel metallacycles .
n 
 coupling of anilide with an acetylide to form a transient ynamine , 
 which rearranges to the keteneimine .
theoretical studies , employing 
 both density functional theory and higher - level ab initio simulations , 
 were applied to understand the bonding and reactivity of the nickel - imido 
 ligand .
unless otherwise stated , all operations were performed in an mbraun 
 lab master drybox under an atmosphere of purified nitrogen or using 
 high - vacuum and standard schlenk techniques under an argon atmosphere .
hexanes , petroleum ether , benzene , and toluene 
 were dried by passage through activated alumina and q-5 columns .
benzene - d6 , deuterated 
 tetrahydrofuran ( thf - d8 ) , and cd2cl2 were purchased , degassed , and dried over cah2 or activated 4  molecular sieves and vacuum transferred .
celite , 
 alumina , and 4  molecular sieves were activated under vacuum 
 overnight at a temperature above 180 c .
anhydrous solvents such 
 as thf and diethyl ether were purchased from acros organics or fischer , 
 stirred over sodium metal , and filtered through activated alumina .. 
 all other chemicals were used as received .
o = c = cph2 , [ ( dtbpe)ni(-cl)]2 , ( dtbpe)ni(cod ) , ( dtbpe)ni{nh(2,6-pr2c6h3 ) } , ( dtbpe)ni = n(2,6-pr2c6h3 ) 
 ( 1 ) , ( dtbpe)ni = n(mes ) 
 ( 2 ) , mes*o radical , mesn = nmes , ( dtbpe)nicl2 , and ( dtbpe)ni = n(1-ad ) 
 ( 3 ) were prepared according 
 to the literature . for complex 1 an alternative , and 
 optimized , procedure is reported below .
infrared data 
 ( fluorolube mulls or solution , caf2 plates or in nujol 
 mulls , kbr plates ) were measured by using a nicolet 670-ft - ir instrument . 

h , c , f , and p nmr 
 spectra were recorded on bruker 500 and 400 mhz nmr spectrometers .
h and c nmr are reported with reference to solvent 
 resonances ( residual c6d5h in c6d6 , 7.16 and 128.0 ppm ; residual chdcl2 in cd2cl2 , 5.32 and 53.8 ppm ; residual proteo thf in 
 thf - d8 , 1.73 and 3.58 ppm , and 65.6 and 
 23.5 ppm ) .
f nmr spectra were reported with respect to 
 external ccl3f ( 0 ppm ) .
p nmr spectra were 
 reported with respect to external 85% h3po4 ( 0 
 ppm ) .
x - ray diffraction data were collected on a siemens platform goniometer 
 with a charged coupled device ( ccd ) detector .
structures were solved 
 by direct or patterson methods using the shelxtl ( version 5.1 ) program 
 library ( g. sheldrick , bruker analytical x - ray systems , madison , wi ) .
a mixture of ( dtbpe)ni{nh(2,6- pr2c6h3 ) } ( 55 mg , 0.1 mmol ) 
 and mes*o ( 28 mg , 0.1 mmol ) in 10 ml of c6h6 was placed in a 20 ml scintillation vial and stirred 
 for 12 h. after removal of volatiles under reduced pressure , the residual 
 solids were extracted with hexanes .
the hexanes solution was concentrated , 
 filtered through a plug of celite , and cooled to 35 c 
 to generate pure green crystals of 1 ( 50 mg , 90% ) .
a mixture of 1 ( 82.8 mg , 0.15 mmol ) , hsnbu3 ( 43.6 mg , 0.15 mmol ) , and c6h6 ( 10 ml ) was placed in a schlenk tube and heated at 
 75 c for 12 h. the mixture gradually turned color from emerald 
 green to dark red . after the mixture cooled to room temperature , the 
 volatiles were removed under reduced pressure , and the solids were 
 extracted with toluene .
after filtration , crystallization occurred 
 by cooling a concentrated toluene solution stored at 35 c 
 over a period of 12 d. red crystals were separated by filtration 
 and dried under reduced pressure to yield analytically pure product , 
 ( dtbpe)ni{nh(2,6- pr2c6h3 ) } ( 68 mg , 82% ) .
was dissolved 1 ( 90 mg , 0.163 mmol ) 
 in 15 ml of et2o , and the green solution was cooled to 
 35 c . to the cold solution was added dropwise 5 ml of 
 an et2o solution containing hccph ( 17 mg , 0.167 
 mmol ) , which caused darkening of the solution over a period of 2 h 
 after reaching room temperature .
the reaction mixture was allowed 
 to stir for an additional 3 h at room temperature .
the dark red solution 
 was filtered , concentrated , and cooled to 35 c overnight 
 to afford ( dtbpe)ni{nh(2,6-pr2c6h3)}(ccph ) ( 4 ) in two crops 
 as dark red crystals / powder , which was filtered , washed with cold 
 petroleum ether , and dried under reduced pressure ( 96 mg , 0.147 mmol , 
 90% yield ) .
h nmr ( 22 c , 500.1 mhz , cd2cl2 ) :  6.95 ( m , aryl , 3 h ) , 6.80 ( d , aryl , 2 h ) , 
 6.49 ( m , aryl , 3 h ) , 4.01 ( sept , ch(ch3)2 , 2 h ) , 1.96 ( br , nh , 1 h ) , 1.88 ( m , 
 ch2 , 2 h ) , 1.73 ( m , ch2 , 2 h ) , 1.56 ( d , bu , 36 
 h , jhp = 13 hz ) , 1.29 ( d , ch(ch3)2 , 6 h ) , 1.16 ( d , ch(ch3)2 , 6 h ) . c{h } nmr 
 ( 22 c , 125.8 mhz , cd2cl2 ) :  158.21 
 ( s , aryl ) , 139.12 ( s , aryl ) , 131.04 ( s , aryl ) , 129.59 ( s , aryl ) , 127.49 
 ( s , aryl ) , 124.35 ( s , aryl ) , 121.83 ( s , aryl ) , 114.41 ( s , aryl ) , 113.85 
 ( d , niccph , jcptrans = 
 21 hz ) , 106.80 ( dd , niccph , jcptrans = 96 hz , jcpcis = 41 hz ) , 
 36.75 ( s , ch(ch3)2 ) , 36.62 
 ( s , ch(ch3)2 ) , 36.11 ( s , c(ch3)3 ) , 36.04 ( s , c(ch3)3 ) , 30.91 ( s , c(ch3)3 ) , 30.70 ( s , c(ch3)3 ) , 28.36 ( s , ch(ch3)2 ) , 25.04 ( s , ch(ch3)2 ) , 24.68 ( t , ch2ch2 , jcp = 15 hz ) , 22.74 ( s , ch(ch3)2 ) , 21.22 ( t , ch2ch2 , jcp = 15 hz ) .
p{h } nmr ( 22 c , 
 202.4 mhz , cd2cl2 ) :  82.80 ( d , jpp = 26 hz ) , 68.84 ( d , , jpp = 26 hz ) .
calcd for c38h64nnip2 : c , 69.73 ; h , 9.70 ; n , 2.14 . found : c , 68.83 ; h , 9.63 ; n , 
 2.00% . a 25 ml round - bottom flask was charged with 2 ( 83 mg , 0.163 mmol ) and 12 ml of et2o , and then it was 
 cooled to 35 c . a similarly cold 2 ml solution of hccph 
 ( 17 mg , 0.163 mmol ) was added dropwise to the nickel solution causing 
 a color change to red - orange .
after it was stirred for 45 min at room 
 temperature , the solution was filtered and then cooled to 35 
 c overnight to give dark red crystals of 5 ( 84 
 mg , 0.132 mmol , 81% ) .
c6d6 ) :  7.23 ( t , c6h5 , 2 h ) , 7.19 ( s , ch2me3 , 2 h ) , 7.14 ( d , c6h5 , 2 h ) , 6.93 ( t , c6h5 , 1 h ) , 
 2.82 ( s , ch3 , 6 h ) , 2.49 ( s , ch3 , 3 h ) , 1.44 ( d , ( ch3)3 , 18 h ) , 1.35 ( d , ( ch3)3 , 18 h ) , 1.13 ( d , ch2 , 4 h ) . c{h } nmr ( 22 c , 125.8 mhz , c6d6 ) :  156.7 ( t , niccph , jcp = 4.2 hz ) , 130.3 ( s , ar ) , 
 128.4 ( s , ar ) , 127.6 ( s , ar ) , 127.4 
 ( s , ar ) , 127.1 ( s , ar ) , 126.6 ( s , ar ) , 123.6 ( s , ar ) , 121.3 ( s , ar ) , 112.0 ( dd , niccph , jcp = 2 , 23 hz ) , 35.8 , 34.5 ( m , c(ch3)3 , jpc = 16.3 hz ) , 35.3 ( m , c(ch3)3 , jpc = 9.6 hz ) , 29.9 ( d , ( ch3)3 , jpc = 3 hz ) , 29.7 ( d , ( ch3)3 , jpc = 3.8 
 hz ) , 23.7 ( t , ch2 , jpc = 15 hz ) , 20.5 ( s , ch3 ) , 19.6 
 ( s , ch3 ) . the nh resonance 
 could not be located .
p{h } nmr ( 22 c , 
 202.4 mhz , c6d6 ) :  81.0 ( d , jpp = 25.2 ) , 67.8 ( d , jpp = 
 25.2 ) .
ir ( nujol , kbr ) : 2092(s cc ) , 1591(m ) , 1294(w ) , 
 1236(s ) , 1178(m ) , 1151(m ) , 1020(m ) , 851(s ) , 812(w ) , 785(w ) , 755(s ) , 
 722(s ) , 693(m ) , 683(m ) , 603(w ) , 500(m ) , 454(w ) cm . anal .
calcd for c35h57ninp2 : c , 
 68.64 ; h , 9.38 ; n , 2.29 . found : c , 68.56 ; h , 9.49 ; n , 2.20% . a 25 
 ml round - bottom flask was charged with 3 ( 31 mg , 0.0589 
 mmol ) and 10 ml of et2o , and then it was cooled to 35 

a similarly cold 2 ml solution of hccph ( 6 mg , 0.0589 
 mmol ) was added dropwise to the nickel solution causing a color change 
 to orange - yellow . after it was stirred for 90 min at room temperature , 
 the solution was filtered , concentrated to 3 ml , and cooled to 35 
 c overnight to give dark orange crystals of 6 ( 32 
 mg , 0.051 mmol , 86% ) .
h nmr ( 22 c , 500 mhz , c6d6 ) :  7.47 ( d , ph , 2 h ) , 
 7.26 ( t , ph , 2 h ) , 7.03 ( t , ph , 
 1 h ) , 2.03 ( s , c10h15 , 6 h ) , 
 1.96 ( s , c10h15 , 3 h ) , 1.38 
 ( d , c10h15 , 3 h ) , 1.36 ( d , 
 c10h15 , 3 h ) , 1.33 ( m , ch2 , 4 h ) , 1.25 ( d , ( ch3)3 , 18 h ) , 1.10 ( d , ( ch3)3 , 18 h ) , c{h } nmr ( 22 c , 125.8 
 mhz , c6d6 ) :  132.6 ( s , ph ) , 131.6 ( s , ph ) , 129.6 ( s , ar ) , 
 127.1 ( s , ph ) , 114.7 ( d , niccph , jcp = 21 hz ) , 100.6 ( dd , niccph , jcp = 91 , 34 hz ) , 43.6 ( t , ad , jpc = 6.8 hz ) , 38.4 ( s , ad ) , 35.4 ( m , c(ch3)3 ) , 32.5 , ( s , ad ) , 30.6 ( d , ( ch3)3 , jpc = 6.3 hz ) , 
 30.2 ( d , ( ch3)3 , jpc = 6.3 hz ) , 23.9 ( m , ch2 ) . the nh resonance could not be located . p{h } nmr ( 22 c , 202.4 mhz , c6d6 ) :  98.8 ( d , jpp = 47.1 hz ) , 90.2 
 ( d , jpp = 47.1 hz ) .
ir ( nujol , kbr ) : 2153(s 
 cc ) , 1589(s ) , 1307(w ) , 1260(w ) , 1179(s ) , 1095(m ) , 
 1067(w ) , 1019(m ) , 935(w ) , 850(w ) , 813(s ) , 755(s ) , 695(m ) , 673(m ) , 
 658(w ) cm .
a 25 ml round - bottom flask was charged with 4 ( 78 
 mg , 0.0141 mmol ) , attached to an adapter , and evacuated .
carbon monoxide 
 ( 1 atm ) was introduced into the vessel at 78 c .
examination 
 of the reaction mixture by p nmr spectroscopy revealed 
 clean formation of ( dtbpe)ni(co)2 along with one major organic product ( observed by h 
 nmr spectroscopy ) .
after 30 min , the gas was evacuated , the flask 
 was opened in air , and the solvent was removed under reduced pressure . 

the phch = c = n(2,6-pr2c6h3 ) residue was passed through a short 
 column of silica with hexanes / etoac ( 4:1 ) as the eluent to provide 
 a viscous oil ( 19 mg , 0.68 mmol , 48% ) .
h nmr ( 22 c , 
 500.1 mhz , cd2cl2 ) :  6.97 ( s , aryl , 3 
 h ) , 5.65 ( s , ch ) , 3.65 ( sept , ch(ch3)2 , 2 h ) , 1.70 ( d , ch(ch3)2 , 12 h ) .
c{h } nmr 
 ( 22 c , 125.8 mhz , cd2cl2 ) :  178.2 
 ( s , ccn ) , 141.2 ( s , aryl ) , 134.1 ( s , aryl ) , 127.3 
 ( s , aryl ) , 124.6 ( s , aryl ) , 56.2 ( s , ccn ) , 28.2 ( s , ch(ch3)2 ) , 23.5 ( s , ch(ch3)2 ) . gc / ms ( m / z ) : 214 ( m ) , 176 , 130 , 91 . in a vial was dissolved 1 ( 100 mg , 
 0.181 mmol ) in 10 ml of et2o , and to the green solution 
 was added dropwise o = c = cph2 in 5 ml of et2o , causing a rapid color change from green to an intense orange - red . 

after 10 min red precipitate began to form , and after allowing the 
 reaction to stir for an additional 40 min , the mixture was cooled 
 to 35 c for 20 min , filtered , and the solids washed 
 with cold et2o .
the red solid was dried under reduced pressure 
 to afford crude ( dtbpe)ni{o , c : oc = n(2,6-pr2c6h3)cph2 } ( 127 mg , 0.172 mmol , 95% yield ) .
analytically pure 7 was obtained by dissolving the solids in a minimum of ch2cl2 , filtering , layering carefully with excess 
 et2o , and cooling the solution to 35 c for 
 1 d. large dark red blocks of 7 were collected via filtration , 
 washed with cold et2o , and dried under vacuum ( 95 mg , 0.127 
 mmol , 70% yield ) .
h nmr ( 22 c , 500.1 mhz , cd2cl2 ) :  8.37 ( d , aryl , 4 h ) , 7.24 ( t , aryl , 
 4 h ) , 7.13 ( t , aryl , 2 h ) , 6.85 ( d , aryl , 2 h ) , 6.74 ( t , aryl , 1 h ) , 
 2.87 ( sept , ch(ch3)2 , 2 h ) , 
 1.80 ( m , ch2 , 2 h ) , 1.48 ( m , ch2 , 2 h ) , 1.44 ( d , bu , 18 h , jhp = 15 hz ) , 1.08 ( d , bu , 18 h , jhp = 13 hz ) , 0.99 ( br , ch(ch3)2 , 6 h ) , 0.86 ( br , ch(ch3)2 , 6 h ) .
c{h } nmr ( 22 c , 125.8 mhz , 
 cd2cl2 ) :  175.7 ( s , oc = n(2,6-pr2c6h3 ) ) , 150.2 ( s , aryl ) , 145.2 ( s , aryl ) , 140.5 ( s , aryl ) , 
 130.6 ( s , aryl ) , 127.4 ( s , aryl ) , 124.0 ( s , aryl ) , 121.5 ( s , aryl ) , 
 120.6 ( s , aryl ) , 35.79 ( cph2 ) , 35.69 ( s , ch(ch3)2 ) , 35.47 ( br , c(ch3)3 ) , 30.63 ( s , c(ch3)3 ) , 30.40 ( s , c(ch3)3 ) , 28.15 ( s , ch(ch3)2 ) , 25.24 ( m , ch2ch2 , jcp = 13 hz ) , 23.50 ( br , 
 ch(ch3)2 ) , 18.05 ( br , ch2ch2 ) . p{h } nmr ( 22 c , 202.4 mhz , cd2cl2 ) :  77.49 ( d , jpp = 13 
 hz ) , 66.21 ( d , jpp = 13 hz ) .
ir ( caf2 , fluorolube mull ) : 3047 ( w ) , 2987 ( w ) , 2953 ( w ) , 2899 ( w ) , 
 1603 ( s , cn ) , 1577 ( s ) , 1482 ( m ) , 1468 ( m ) , 1429 
 ( m ) , 1391 ( w ) , 1357 ( w ) cm .
calcd for c44h67nnip2o : c , 70.78 ; h , 9.04 ; n , 1.88 . 
 found : c , 70.38 ; h , 9.33 ; n , 1.92% . a 25 ml round - bottom flask was charged with 2 ( 48 mg , 0.094 mmol ) and 5 ml of et2o , and then 
 it was cooled to 35 c . a similarly cold solution of 
 diphenylketene ( 18 mg , 0.094 mmol ) in 4 ml of et2o was 
 added dropwise to 2 , causing a color change to red - orange . 

the solution was filtered after 1.5 h of stirring at room temperature , 
 then concentrated to ca .
4 ml and cooled to give orange crystals of 8 ( 51 mg , 0.068 mmol , 72% ) .
h nmr ( 22 c , 
 500 mhz , cd2cl2 ) :  8.22 ( d , ph , 4 h ) , 7.13 ( t , ph , 4 h ) , 7.06 ( t , ph , 2 h ) , 6.58 ( s , ch2me3 , 2 
 h ) , 2.07 ( s , ch3 , 3 h ) , 1.79 ( s , ch3 , 6 h ) , 1.34 ( m , ch2 , 4 h ) , 1.23 ( d , ( ch3)3 , 18 
 h ) , 1.01 ( d , ( ch3)3 , 18 h ) .
c{h } nmr ( 22 c , 125.8 mhz , cd2cl2 ) :  170.2 ( s , co ) , 133.8 ( s , ar ) , 130.3 ( s , ar ) , 129.8 ( s , ar ) , 127.3 ( s , ar ) , 127.2 ( s , ar ) , 
 123.7 ( s , ar ) , 122.6 ( s , ar ) , 121.3 
 ( s , ar ) , 38.1 ( t , cph2 , jpc = 15 hz ) , 35.4 ( m , c(ch3)3 ) , 30.4 ( d , ( ch3)3 , jpc = 4.6 hz ) , 30.1 ( d , ( ch3)3 , jpc = 3.7 
 hz ) , 22.7 ( m , ch2 ) , 22.1 ( s , ch3 ) , 18.0 ( s , ch3 ) .
p{h } nmr ( 22 c , 202.4 mhz , cd2cl2 ) :  78.5 ( d , jpp = 12.4 
 hz ) , 66.9 ( d , jpp = 12.4 hz ) .
ir ( nujol , 
 kbr ) : 1594(m ) , 1564(s , cn ) , 1266(m ) , 1232(w ) , 1180(m ) , 
 1022(m ) , 859(w ) , 837(m ) , 799(w ) , 786(w ) , 738(w ) 716(m ) , 694(s ) , 673(m ) , 
 651(w ) cm .
calcd for c40h61ninop2 : c , 69.37 ; h , 8.88 ; n , 2.02 . found : c , 
 68.87 ; h , 8.22 ; n , 2.01% .
a 25 
 ml round - bottom flask was charged with 3 ( 95 mg , 0.180 
 mmol ) and 7 ml of et2o , and then it was cooled to 35 

c . a similarly cold solution of diphenylketene ( 35 mg , 0.180 
 mmol ) in 2 ml of et2o was added dropwise to 3 , causing little visible color change .
the solution was filtered 
 after 1.5 h of stirring at room temperature , then concentrated to 
 ca . 4 ml and cooled to give dark red blocks of 9 ( 87 
 mg , 0.139 mmol , 77% ) .
h nmr ( 22 c , 500 mhz , c6d6 ) :  8.55 ( d , ph , 4 h ) , 
 7.24 ( t , ph , 4 h ) , 7.10 ( t , ph , 
 2 h ) , 2.38 ( s , c10h15 , 6 h ) , 
 2.18 ( s , c10h15 , 3 h ) , 1.85 
 ( d , c10h15 , 3 h ) , 1.78 ( d , 
 c10h15 , 3 h ) , 1.16 ( d , ( ch3)3 , 18 h ) , 1.13 ( m , ch2 , 4 h ) , 0.88 ( d , ( ch3)3 , 18 h ) , c{h } nmr ( 22 c , 125.8 
 mhz , c6d6 ) :  179 ( br , co ) , 141.6 ( s , ph ) , 131 ( s , ph ) , 
 127.0 ( s , ph ) , 126.3 ( s , ph ) , 48.6 
 ( t , ad , jpc = 6.2 hz ) , 
 38.6 ( s , ad ) , 36.4 ( t , cph2 , jpc = 12 hz ) , 35.2 ( m , c(ch3)3 ) , 32.4 , ( s , ad ) , 30.2 ( d , ( ch3)3 , jpc = 6.5 hz ) , 29.3 ( d , ( ch3)3 , jpc = 6.7 hz ) , 22.1 ( m , ch2 ) . p{h } nmr ( 22 c , 202.4 
 mhz , c6d6 ) :  77.5 ( d , jpp = 19.8 hz ) , 62.4 ( d , jpp = 19.8 hz ) .
ir ( caf2 , fluorolube ) : 1559(s ) , 1469(s ) , 
 1444(m ) , 1385(w ) , 1361(s ) cm . in a vial
was dissolved 1 ( 80 
 mg , 0.145 mmol ) in 8 ml of toluene , and the green solution was cooled 
 to 35 c . to the cold solution was added dropwise 3 ml 
 of a toluene solution containing o = c = nch2ph ( 21 mg , 0.158 mmol ) causing darkening of the solution over a period 
 of 2 h at room temperature and with precipitation of green solid . 

the reaction mixture was allowed to stir for an additional 3 h at 
 room temperature .
the dark solution was concentrated , cooled to 35 
 c overnight , and then filtered ; the green solids were washed 
 with cold et2o and dried under vacuum to afford crude ( dtbpe)ni{o , c : oc = nch2phn(2,6-pr2c6h3 ) } 
 ( 10 ) ( 89 mg , 131 mmol , 90% yield ) .
the solids were dissolved 
 in a minimum of ch2cl2 , filtered , layered carefully 
 with excess et2o , and cooled to 35 c for 
 2 d. large dark green blocks of 10 were collected via 
 filtration , washed with et2o , and dried under vacuum ( 76 
 mg , 0.110 mmol , 76% yield ) .
h nmr ( 22 c , 500.1 mhz , 
 cd2cl2 ) :  7.21 ( d , aryl , 2 h ) , 7.14 ( t , 
 aryl , 2 h ) , 7.01 ( t , aryl , 1 h ) , 6.93 ( s , aryl , 3 h ) , 4.26 ( sept , 
 ch(ch3)2 , 2 h ) , 4.10 ( s , ch2ph , 2 h ) , 1.65 ( m , ch2ch2 , 4 h ) , 1.58 ( d , bu , 18 h , jhp = 13 hz ) , 
 1.24 ( d , ch(ch3)2 , 12 h ) , 1.23 
 ( d , bu , 18 h , jhp = 13 hz ) . c{h } nmr ( 22 c , 
 125.8 mhz , cd2cl2 ) :  173.4 ( s , oc = nch2ph ) , 147.2 ( br , aryl ) , 146.8 ( s , 
 aryl ) , 128.5 ( s , aryl ) , 127.5 ( s , aryl ) , 124.8 ( s , aryl ) , 123.3 ( s , 
 aryl ) , 122.6 ( s , aryl ) , 47.23 ( s , ch2ph ) , 
 36.01 ( d , c(ch3)3 , jcp = 15 hz ) , 35.51 ( d , c(ch3)3 , jcp = 15 hz ) , 30.97 
 ( s , ch(ch3)2 ) , 30.40 ( s , c(ch3)3 ) , 29.23 ( s , ch(ch3)2 ) , 26.28 ( s , ch(ch3)2 ) , 24.24 ( m , ch2ch2 ) , 23.28 ( s , ch(ch3)2 ) , 21.05 ( m , ch2ch2 ) .
p{h } nmr ( 22 c , 
 202.4 mhz , cd2cl2 ) :  86.01 ( d , jpp = 27 hz ) , 83.31 ( d , jpp = 27 hz ) .
calcd for c38h64n2nip2o : c , 66.57 ; h , 9.41 ; n , 4.09 . found : c , 66.17 ; 
 h , 9.49 ; n , 3.71% . in 
 a vial was dissolved 1 ( 138 mg , 0.250 mmol ) in 10 ml 
 of toluene , and the solution was cooled to 35 c .
to 
 the green solution was added dropwise a similarly cold solution of 
 phhc = o ( 32 mg , 0.300 mmol ) in 3 ml of toluene .
the solution 
 was stirred for 7 h at room temperature , during which time the color 
 gradually changed from green to brown to finally a dark navy blue 
 color .
the dark solution was filtered , dried under vacuum , extracted 
 with 50 ml of et2o , filtered , concentrated , and cooled 
 to 35 c for 2 d to afford large blue needles and powder 
 of ( dtbpe)ni{o , c : ochphn(2,6-pr2c6h3 ) } 
 ( 11 ) ( 137 mg , 0.207 mmol , 83% yield ) , which was filtered 
 and washed with cold petroleum ether .
h nmr ( 22 c , 500.1 mhz , c6d6 ) :  8.15 ( d , aryl , 2 h ) , 7.39 ( d , aryl , 1 h ) , 
 7.33 ( t , aryl , 1 h ) , 7.18 ( m , aryl , 2 h ) , 7.01 ( d , aryl , 1 h ) , 6.63 
 ( d , aryl , 1 h ) , 4.27 ( sept , ch(ch3)2 , 2 h ) , 1.94 ( d , ch(ch3)2 , 3 h ) , 1.88 ( d , ch(ch3)2 , 
 3 h ) , 1.57 ( d , bu , 9 h , jhp = 13 hz ) , 1.45 ( d , bu , 
 9 h , jhp = 12 hz ) , 1.30 ( d , bu , 9 h , jhp = 12 hz ) , 
 1.27 ( d , ch(ch3)2 , 3 h ) , 1.09 
 ( br , ch2 , 2 h ) , 0.88 ( br , ch2 , 2 h ) , 0.73 ( d , bu , 9 h , jhp = 12 hz ) , 0.47 ( d , ch(ch3)2 , 3 h ) . note : the ochph 
 proton could not be located in the h nmr spectrum . c{h } nmr ( 22 c , 125.8 mhz , c6d6 ) :  157.4 ( s ) , 152.8 ( s ) , 148.9 ( s , aryl ) , 147.6 
 ( s , aryl ) , 127.5 ( s , aryl ) , 126.5 ( s , aryl ) , 124.3 ( s , aryl ) , 122.8 
 ( s , aryl ) , 122.6 ( s , aryl ) , 114.3 ( s , aryl ) , 35.32 ( d , c(ch3)3 , jcp = 6 
 hz ) , 35.17 ( d , c(ch3)3 , jcp = 6 hz ) , 34.78 ( d , c(ch3)3 , jcp = 13 hz ) , 34.37 
 ( d , c(ch3)3 , jcp < 3 hz ) , 31.02 ( br ) , 30.47 ( br ) , 30.24 ( br ) , 29.92 
 ( br ) , 29.79 ( br ) , 27.46 ( s , ch(ch3)2 ) , 26.84 ( s , ch(ch3)2 ) , 25.50 ( s , ch(ch3)2 ) , 23.84 
 ( s , ch(ch3)2 ) , 23.59 ( m , ch2ch2 ) , 19.59 ( m , ch2ch2 ) . p{h } nmr ( 22 c , 202.4 mhz , c6d6 ) :  75.45 ( d , jpp = 6 hz ) , 69.29 
 ( d , jpp = 7 hz ) .
calcd for c37h63nnip2o : c , 67.48 ; h , 9.64 ; n , 2.13 . 
 found : c , 67.22 ; h , 9.78 ; n , 2.10% . in a schlenk 
 flask equipped with a stir bar was dissolved 1 ( 126 mg , 
 0.228 mmol ) in et2o , and the solution was degassed and 
 cooled to 78 c . to the cold green solution
was added 
 excess co2 ( 25 c , 1 atm ) for several minutes , and 
 the solution was allowed to slowly reach room temperature . upon reaching 
 0 c
the solution changed slowly to a pale yellow , and a pale 
 yellow precipitate formed upon reaching room temperature .
the solution 
 was allowed to stir for an additional 2 h at room temperature , the 
 solvent was removed under reduced pressure , and the flask was taken 
 into the box .
the solids were filtered and washed with cold et2o / petroleum ether ( 2:1 ) , and the solids were dried under vacuum 
 to afford crude ( dtbpe)ni{o , c : oc(o)n(2,6-pr2c6h3 ) } 
 ( 12 ) as a pale yellow solid ( 128 mg , 0.214 mmol , 94% 
 yield ) .
analytically pure complex 12 can be obtained 
 by dissolving the crude product in a minimum of ch2cl2 , filtering the golden brown solution , layering the filtrate 
 carefully with excess et2o , and cooling the solution to 
 35 c for 2 d ( 2 crops , 71 mg , 0.119 mmol , 52% yield ) .
h nmr ( 22 c , 500.1 mhz , cd2cl2 ) :  6.97 ( m , aryl , 3 h ) , 4.00 ( sept , ch(ch3)2 , 2 h ) , 1.70 ( m , ch2 , 4 h ) , 1.65 ( d , bu , 18 h , jhp = 13 hz ) , 1.25 ( d , ch(ch3)2 , 6 h ) , 1.21 ( d , bu , 18 
 h , jhp = 13 hz ) , 1.18 ( d , ch(ch3)2 , 6 h ) . c{h } nmr ( 22 c , 125.8 mhz , cd2cl2 ) :  
 170.5 ( br , co2 ) , 147.1 ( s , aryl ) , 144.2 
 ( s , aryl ) , 124.3 ( s , aryl ) , 122.6 ( s , aryl ) , 36.91 ( d , c(ch3)3 , jcp = 16 
 hz ) , 35.65 ( d , c(ch3)3 , jcp = 16 hz ) , 30.35 ( s , c(ch3)3 ) , 29.48 ( s , ch(ch3)2 ) , 25.82 ( s , ch(ch3)2 ) , 24.24 ( m , ch2ch2 , jcp = 12 hz ) , 22.86 ( s , 
 ch(ch3)2 ) , 21.23 ( m , ch2ch2 , jcp = 11 hz ) . p{h } nmr ( 22 c , 
 202.4 mhz , cd2cl2 ) :  89.71 ( d , jpp = 28 hz ) , 85.67 ( d , jpp = 28 hz ) .
ir ( caf2 , fluorolube ) : 2955 ( w ) , 1617 
 ( s , co ) , 1588 ( m ) , 1468 ( m ) , 1431 ( m ) , 1371 ( w ) , 
 1360 ( w ) , 1321 ( w ) cm .
elemental analysis and 
 single - crystal x - ray diffraction methods were consistent with complex 
 ( dtbpe)ni{-oc(o)n(2,6-(chme2)2c6h3 ) } retaining one molecule of ch2cl2 .
calcd for c32h53cl2nnip2 : c , 56.41 ; h , 8.73 ; n , 2.06 . found : c , 57.72 ; 
 h , 8.53 ; n , 2.05% . a 50 ml round - bottom 
 flask was charged with 3 ( 80 mg , 0.152 mmol ) , attached 
 to an adapter , and evacuated .
petroleum ether ( 10 ml ) was vacuum transferred 
 into the flask , and 1 atm co2 was introduced .
the solution 
 became slightly red then lightened to a pale yellow , and a small amount 
 of precipitate formed .
after 30 min the co2 was removed , 
 and the solution was dried under reduced pressure . the solids were 
 extracted with 10 ml of et2o , filtered , and cooled to 35 
 c to provide yellow crystals of 13 ( 58 mg , 0.094 
 mmol , 62% ) .
h nmr ( 22 c , 500 mhz , cd2cl2 ) :  2.29 ( d , c10h15 , 3 h ) , 2.01 ( s , c10h15 , 6 h ) , 1.68 ( d , c10h15 , 3 h ) , 1.64 ( d , c10h15 , 3 
 h ) , 1.54 ( d , ( ch3)3 , 36 h ) , 
 1.52 ( d , ch2 , 4 h ) . c{h } nmr ( 22 c , 125.8 mhz , cd2cl2 ) :  157.9 ( s , co2 ) , 40.9 ( s , ad ) , 38.4 ( s , ad ) , 37.6 ( t , c(ch3)3 , jpc = 6.0 
 hz ) , 31.8 , ( s , ad ) , 31.1 ( d , ( ch3)3 , jpc = 6.2 hz ) , 
 23.1 ( m , ch2 ) . p{h } nmr ( 22 c , 202.4 mhz , cd2cl2 ) :  
 87.7 ( s ) .
ir ( caf2 , fluorolube ) : 1667(s , co ) , 1624(s , co ) , 1481(w ) , 1465(s ) , 1377(s ) , 1352(s ) 
 cm .
a schlenk tube was charged with 2 ( 76 mg , 0.149 mmol ) 
 and 6 ml of benzene .
the solution was degassed and heated to 80 c 
 for 16 h. the tube was then opened in air , and the solvent was removed 
 under reduced pressure .
azomesitylene was purified on a silica gel 
 column with 5:1 hexanes / etoac as eluent to give red crystals ( 16 mg , 
 0.060 mmol , 40% ) .
h nmr ( 22 c , 500 mhz , cdcl3 ) :  6.95 ( s , c6me3h2 , 2 h ) , 2.40 ( s , c6me3h2 , 6 h ) , 2.32 ( s , c6me3h2 , 3 h ) .
c{h } nmr 
 ( 22 c , 125.8 mhz , cdcl3 ) :  130.3 ( s , mes ) , 129.8 ( s , mes ) , 128.2 ( s , mes ) , 127.6 ( s , mes ) , 22.4 ( s , ch3 ) , 21.7 ( s , ch3 ) .
. computational studies of complex 1 utilized density functional theory ( specifically , oniom(m06/6 - 311+g(d):uff ) ; geometry optimizations 
 started from the reported crystal structure , using the gaussian 09 code . at this 
 optimized geometry ,
multiconfiguration self - consistent field ( mcscf ) computations were performed utilizing the complete 
 active space ( cas ) formalism within the gamess code .
complex ( dtbpe)ni = n(2,6-pr2c6h3 ) 
 ( 1 ) is a green crystalline 
 species that possesses a ni 
n formal bond order of two and 
 should therefore react similar to the carbene complex ( dtbpe)ni = cph2 or phosphinidene ( dtbpe)ni = p(dmp ) . likewise , the imido 
 relatives ( dtbpe)ni = n(mes ) ( 2 , turquoise ) and ( dtbpe)ni = n(1-ad ) ( 3 , 
 red ) can also be prepared , and their chemistry 
 can be similarly explored .
complexes 2 and 3 are obtained by a different route than 1 , using the 
 corresponding organic azide and ni(0 ) precursor ( scheme 1 ) .
complex 1 is synthesized via a one - electron 
 oxidation of the three - coordinate ni(i ) complex ( dtbpe)ni{nh(2,6-pr2c6h3 ) } 
 with the weak oxidant [ c7h7][pf6 ] 
 followed by deprotonation with na{n(sime3)2 } 
 ( scheme 2 ) .
although this protocol is reliable , 
 with each reaction being high yielding , it has two disadvantages : 
 ( i ) it is a stepwise process to remove overall an h atom , and ( ii ) 
 the synthesis of ( dtbpe)ni{nh(2,6-pr2c6h3 ) } involves the use of the precursor 
 [ ( dtbpe)ni(-cl)]2 , a complex 
 prepared in moderate yield from one - electron reduction of ( dtbpe)nicl2 with kc8 ( scheme 2 ) . the use of a ni(i ) precursor 
 is required given that attempts to transmetallate and dehydrohalogenate 
 ( dtbpe)nicl2 with 2 equiv of li{nh(2,6-pr2c6h3 ) } resulted in complicated 
 mixtures that contained traces of 1 and other species 
 such as ( dtbpe)ni{nh(2,6-pr2c6h3)}. to improve 
 the overall yield of [ ( dtbpe)ni(-cl)]2 , we reported 
 a comproportionation reaction using easy - to - prepare ni(0 ) and ni(ii ) 
 complexes , namely , ( dtbpe)ni(cod ) and 
 ( dtbpe)nicl2
. complex [ ( dtbpe)ni(-cl)]2 can be prepared quantitatively from these 
 two reagents ( scheme 2 ) akin to sigman s 
 reported n - heterocyclic carbene complex of ni(i ) .
notably , the synthesis of [ ( dtbpe)ni(-cl)]2 from this reaction does not require isolation or purification of 
 the ni(ii ) and ni(0 ) starting materials since it can be produced by 
 simply adding premixed solutions of ni(cod)2 with dtbpe 
 and nicl2 with dtbpe over several hours .
likewise , to obviate the need for separate oxidation and 
 deprotonation steps , a h atom - abstraction reaction with the radical 
 mes*o ( mes * = 2,4,6-bu3c6h2 ) was used for the direct preparation of 1 .
smith and hillhouse have applied this strategy 
 to prepare cobalt imido and other nickel imido derivatives , respectively . 

consequently , a more convenient route to multigram quantities of imido 1 is shown in scheme 3 via a comproportionation 
 reaction to form [ ( dtbpe)ni(-cl)]2 , followed by 
 transmetalation with li{nh(2,6-pr2c6h3 ) } to yield ( dtbpe)ni{nh(2,6-pr2c6h3 ) } 
 in 92% yield , and then the protocol was completed by h atom abstraction 
 with mes*o .
separation of 1 from the homes byproduct 
 can be achieved by fractional crystallization to provide 1 in 90% yield ( scheme 2 ) .
preliminary reactivity studies confirmed that the imido 
 ligand in 1 can be readily carbonylated with co 
 to form o = c = n(2,6-pr2c6h3 ) or with cnch2ph to form the asymmetric carbodiimido phch2n = c = n(2,6-pr2c6h3 ) . 

similar reactions have been explored with 2 and 3 . in both reactions , the -isocyanate or -carbodiimido intermediate 
 could be isolated .
in addition , [ 2 + 
 2 ] cycloaddition of ethylene across the ni = n bond in 1 has been shown to be a crucial step in the formation of 
 aziridines , via an elusive azametallacyclobutane 
 intermediate .
the 2001 report by 
 mindiola and hillhouse of complex 1 foreshadowed 
 a tremendous upsurge in interest in late transition - metal multiply 
 bonded complexes .
the original crystallographic report has several 
 hallmarks of a complex  engineered  to have sufficient 
 stability to permit solid - state analysis  a bulky bidentate 
 supporting ligand , a sterically hindering imido n substituent , and 
 hints of further stabilization of the -loaded imido nitrogen 
 via resonance with the aryl substituent ( cf .
however , as detailed 
 above , these features belie a remarkable diversity of reactivity for 
 complex 1 encompassing both even- and odd - electron processes 
 as well as reactions involving homolytic ( radical ) and heterolytic 
 ( acid / base ) transformations .
a previous computational study in 2008 , inspired by the original experimental reports 
 by mindiola and hillhouse , indicated that 
 subtle changes to the ligands and substituents markedly affect the 
 computed kinetics and thermodynamics for highly desirable reactions 
 such as c  h bond activation . to this end
, the electronic structure 
 of 1 was scrutinized anew using theoretical methods not 
 feasible on such a large complex when the earlier reports were published .
the pr and bu substituents on the 
 imidoaryl and dtbpe ligands , respectively , were modeled with the universal 
 force field , while the remainder of the 
 complex was described at the m06/6 - 311+g(d ) level of theory .
as expected , good agreement with the reported crystal data was obtained 
 ( coordinates in supporting information ) , 
 and tests with other functionals  pure ( bp86 ) and hybrid ( b3lyp)gave 
 similar results .
the density functional theory - optimized geometry 
 was then used as the basis to analyze the electronic structure of 1 with mcscf techniques employing the cas 
 approximation with active spaces ranging from two - orbital / two - electron 
 ( cas(2,2 ) ) to 14-orbital/14-electron ( cas(14,14 ) ) .
the salient features 
 of the frontier natural orbitals are similar among the various cas 
 calculations ; therefore , focus is given to the pertinent orbitals 
 of the latter , largest active space mcscf simulations .
nimido  and 
  * orbitals that lie within the plane defined by the nip2n coordination plane ( figure 1 ) .
the 
 ca(14,14 ) calculations assign a substantial population to the latter 
 orbital , 0.24 e , and thus the correlating  has a natural orbital occupation 
 number ( noon ) of 1.76 e. these orbitals noons are those computed to deviate 
 most significantly from the archetypal values of 2 and 0 e , and thus the mcscf calculations 
 are indicative of significant -biradical character to the ni - imido 
 moiety , consistent with the ability of 1 to abstract 
 an h atom from hsn(bu)3 .
additionally , 
 the -biradical nature of 1 implies a formal bond 
 order of less than three despite the nearly linear coordination of 
 the imido functionality , which obviously assists the [ 2 + 2 ] cycloaddition 
 and 1,3-dipolar addition chemistry of complex 1 and its 
 congeners . computed frontier 
 orbitals (  and  * ) of complex 1 in the plane 
 defined by the n and the two p atoms .
illustration of the  
 and  * orbitals in 1 perpendicular to the plane 
 defined by the n and two p. hydrogen atoms omitted from figure for 
 clarity .
the in - plane nin / * 
 orbitals just discussed may be contrasted to the perpendicular  
 orbitals ( figure 2 ) . the  orbital ( figure 2 , left ; noon = 1.97 e ) is heavily polarized toward the imido n 
 with small ni character but delocalized onto the imido aryl ring , 
 { 2,6-pr2c6h3}. electron correlation is not of the bond / antibond variety 
 ( cf .
figure 1 ) but with an orbital that has 
 an additional radial node ( figure 2 , right ; noon = 0.03 e ) .
the additional radial 
 node in the right diagram in figure 2 is clear ; 
 note that like its bonding counterpart , there is delocalization of 
 electron density onto the imido 2,6-pr2c6h3 ring for both members of this correlating 
 pair of orbitals .
these mcscf orbitals imply that in this plane the 
 ni  n -bond may be best viewed more as a nimido lone pair , with potentially high nucleophilic or basic reactivity . 
 as with the biradical character
, the bonding analysis suggests a further 
 diminution of the nickel  nitrogen bond order below the ideal 
 value of three that a cursory glance at its linear coordination mode 
 might imply .
computed natural orbitals showing delocalization of -electron 
 density from imido nitrogen of 1 onto the 2,6-pr2c6h3 ring : noon 
 = 1.94 ( left ) and 0.06 e ( right ) .
the final pair of orbitals 
 of interest from the mcscf computations are plotted below ( figure 3 ) .
these orbitals highlight the substantial interaction 
 between the imido n and the -ring of the aryl substituent . 

much of the experimental emphasis on late metal imidos has focused 
 on the steric protection of the metal 
the present computations suggest 
 the great potential to synthetically tune the reactivity of these 
 and related late metal aryl - imido moieties via the introduction of 
 electron - donating and -withdrawing groups onto the meta and para aryl ring positions .
as such , with reduced 
 metal  nitrogen bond orders and synthetic tunability , one may 
 easily envisage derivatives of 1 providing improved routes 
 to c  h bond amination that can complement known metal - catalyzed 
 routes to c 
the observation that the 
 frontier  symmetry orbitals displayed in figures 13 are heavily composed of nimido character suggests that these systems should be quite 
 basic . because the three - coordinate anilide cation [ ( dtbpe)ni = nh(2,6-pr2c6h3)][pf6 ] can be deprotonated by nan(sime3)2 , the pka of the 
 anilide can be coarsely estimated to be less than 30 .
we thus examined phenylacetylene as a substrate because 
 its pka is slightly less than 30 in dimethyl 
 sulfoxide .
accordingly , treatment of 
 the imidos 13 with hccph 
 in et2o results in rapid and clean formation of analytically 
 pure phenylacetylidene 
amides , as red crystals of ( dtbpe)ni{nh(2,6-pr2c6h3)}(ccph ) 
 ( 4 ) , ( dtbpe)ni{nh(mes)}(ccph ) ( 5 ) , and orange crystals of ( dtbpe)ni(nh{1-ad})(ccph ) ( 6 ) , in 81% , 90% , and 86% isolated yields , respectively ( scheme 4 ) .
these results imply that the anilide in [ ( dtbpe)ni = nh(2,6-pr2c6h3)][pf6 ] has a pka from 29 to 35 .
since 
 the imidos 2 and 3 are not prepared by amide 
 deprotonation we do not know their basicity ; however , complex 2 most likely has a similar pkb to 1 .
for 
 example , the h and c nmr spectra of complex 4 reveals the amide ( nh , 1.93 ppm ) and acetylide 
 moieties ( ccph , 107 and 
 114 ppm ) , respectively .
the acetylide -c reveals diagnostic jcp = 96 and 41 hz , respectively , 
 for trans and cis coupling to dtbpe , which are much greater than the 
 coupling observed for the acetylide -c jcp = 21 hz to the trans phosphorus . because 
 of the lack of c2 symmetry , the p nmr spectrum shows a pair of doublets for the dtbpe ligand with 
 the respective jpp values 
 listed in table 1 .
infrared spectra of 5 and 6 further confirm a terminal acetylide ligand with 
 cc = 2091 and 2153 cm , respectively . 

x - ray crystallographic analysis of a single crystal of 4 grown from a saturated et2o solution cooled to 35 
 c reveals a square - planar ni(ii ) complex with cis acetylide 
 and anilide ligands ( figure 4 ) .
n 
 bond length in 4 is significantly longer at 1.933(3 ) 
  than that observed in [ ( dtbpe)ni = n(h)(2,6-pr2c6h3)][pf6 ] ( 1.768(14 )
 ) because of the now filled 
 ni d - orbital ( of b2 symmetry ) in an ideal 
 square planar and c2 symmetric environment .
the lone pair on the anilide nitrogen in 4 can no longer donate to the metal , hence , the pyramidalization . 

solid - state structural diagram of complex 4 ( thermal ellipsoids at 50% probability ) .
one chemically 
 equivalent but crystallographically independent molecule along with 
 two et2o molecules confined in the asymmetric unit were 
 omitted for clarity .
the coupling of amines 
 with alkynes is a relatively difficult , yet synthetically desirable , 
 process for the construction of versatile synthetic precursors .
complexes 46 could be considered analogous to intermediates in a reaction coupling 
 these two organic fragments .
accordingly , 
 reaction of 4 with co ( 78 c , 1 atm ) completely 
 converted this nickel species to the known biscarbonyl complex ( dtbpe)ni(co)2 based on p nmr 
 spectroscopy . the mass spectrum of the major organic was consistent 
 with the reductive elimination of the alkyne and amide ligands ( scheme 4 ) .
ynamines , especially those containing a secondary 
 amine as in the case of phccnhar , are known to tautomerize 
 to keteneimines spontaneously .
the h nmr spectrum of this new organic product revealed a diagnostic 
 vinylic singlet at  5.62 consistent with the rearranged keteneimine 
 phch = c = nar being produced .
it has been shown that nickel 
 complexes supporting a terminal imido ligand can engage in some cycloaddition 
 chemistry , and in some cases , the imido can be completely transferred 
 to ethylene , co , and cnr .
iron and cobalt imidos have been also shown to be reactive with 
 these substrates .
consequently , we examined 
 the reactivity of 46 with other 
 unsaturated , small molecules .
accordingly , diphenylketene ( o = c = cph2 ) smoothly reacted to afford 
 the oxy metallacyclobutane species ( dtbpe)ni{o , c : oc(cph2)=n(r ) } shown in scheme 5 ( r = 2,6-pr2c6h3 ( 7 ) , mes ( 8) , 
 1-ad ( 9 ) ) .
formation of 79 is nearly quantitative , although isolated yields can range from 
 95 to 72% .
the p nmr spectra reveal two inequivalent 
 phosphorus nuclei with their corresponding jpp values between 13 and 20 hz ( table 1 ) . because the lowest unoccupied molecular orbital of o = c = cph2 is augmented mostly with o 
c  * character , one would expect [ 2 + 2]-cycloaddition to take 
 place by o = c addition across the ni = n to form an aza 
 oxy metallacyclobutane .
indeed , the c nmr spectra of 79 show a highly deshielded resonance 
 at 170 ppm in accord with a carbon atom possessing electron - withdrawing 
 groups ( table 1 ) . to determine unambiguously 
 the type of metallacycle formed , a single - crystal x - ray diffraction 
 study of 7 was performed . to our surprise ,
the solid - state 
 structure of 7 revealed a square planar oxy nickel cyclobutane 
 species , whereby the imido had virtually undergone insertion into 
 the electrophilic carbon of the ketene ( figure 5 ) .
n = 1.280(2 )  is consistent with a double 
 bond , while all metrical parameters in the metallacycle suggest single 
 bonds in the nioc2 ring .
formation of 79 implies that cycloaddition across the ni = n took place , 
 though it can not be distinguished whether initial o = c versus 
 c = c bond addition took place based on these data .
o , c - cycloaddition of the ketene group across ni = n 
 to form metallacycle a is the preferred pathway given 
 the frontier orbitals of o = c = cph2 . from a , two routes can take place : ( i ) homo or heterolytic ni 
n 
 bond cleavage and rotation about the c  o bond or ( ii ) retrocycloaddition 
 to form a nickel  oxo b and keteneimine rn = c = cph2 , which can add across the c = c bond to form the nioc2 ring . the propensity of a terminal nickel 
alternatively , ketene can add via the c  c 
  * bond to form the azametallacyclobutane c , which , 
 analogously to a , can undergo two similar pathways , namely , 
 ni 
n bond rupture or retrocycloaddition ( via the nickel carbene d ) , to ultimately form the ni 
solid - state structural diagram of complexes 7 ( left ) 
 and 10 ( right ) with thermal ellipsoids at 50% probability . 

one et2o molecule in the asymmetric unit was omitted from 
 the structure of 10 . to isolate an aza oxy metallacyclobutane species such 
 as a , the reactivity of 1 with the less 
 sterically hindered isocyanate o = c = nch2ph 
 was investigated .
accordingly , it was found that the o , c - cycloaddition product ( dtbpe)ni{o , c : oc = nch2phn(2,6-pr2c6h3 ) } ( 10 ) could be isolated in 90% yield as green - colored crystals 
 ( scheme 5 ) .
salient spectroscopic data for 10 are shown in table 1 , with the most 
 notable feature being the c = n resonance at 173.4 ppm in the c nmr spectrum . like complexes 49 ,
compound 10 also lacks c2 symmetry but has a stronger jpp value of 27 hz when compared to complex 7 , as a result of the phosphines of the dtbpe ligand not being pushed 
 back due to the sterically congested cph2 group .
therefore , 
 one would expect less of a distortion from a square planar geometry . 

a solid - state structure of 10 confirms formation of a 
 rare example of an aza oxy nickel cyclobutane square planar complex 
 with an elongated ni 
n bond ( 1.939(6 )  ) as compared to 
 the imido precursor 1 ( figure 5 ) . examples of n , o - bound ureates 
 resulting from isocyanate cycloaddition across early transition metals 
 having a terminal imido ligand have been reported . the ni  o bond length ( 1.864(5 )
. similar cycloaddition chemistry of 1 was observed 
 with benzaldehyde , which gave the aza oxy nickel cyclobutane complex 
 ( dtbpe)ni{o , c : ochphn(2,6-pr2c6h3 ) } ( 11 ) in 83% yield .
overall , the nmr spectroscopic features 
 of 11 are similar to those of complex 10 , and the solid - state structure confirms a square planar aza oxy 
 nickel cyclobutane scaffold with ni 
n = 1.924(4 )  and 
 ni  o = 1.842(3 )  distances that compare favorably to 
 those of 10 ( figure 6 ) .
solid - state 
 structural diagram ( thermal ellipsoids at 50% probability ) of complexes 11 ( left ) and 12 ( right ) .
one et2o 
 molecule for 11 and one ch2cl2 molecule 
 for 12 , found in the asymmetric unit , are excluded for 
 clarity .
exposure of a cold et2o solution of 1 to 1 atm co2 resulted in 
 the gradual precipitation of a pale yellow solid containing ( dtbpe)ni{o , c : oc(o)n(2,6-pr2c6h3 ) } ( 12 ) ( scheme 5 ) .
the reaction is quantitative by p nmr spectroscopy , and yellow solids of 12 can be isolated 
 in 94% yield .
in addition to displaying two inequivalent phosphine 
 resonances ( jpp = 28 hz ) akin 
 to the other nickel metallacycles , the most salient spectroscopic 
 feature in 12 is the presence of a broad c nmr resonance at 170.5 ppm in accord with a [ 2 + 2]-cycloadded 
 co2 carbon .
likewise , co = 1617 cm in the infrared region corroborates the presence 
 of this carbonyl functionality .
a solid - state structure of 12 is shown in figure 6 and again depicts a 
 square planar nickel complex possessing a metallalactam or carbamate 
 framework due to [ 2 + 2 ] cycloaddition of co2 across the 
 ni = n bond with distances of ni 
o = 1.9334(15 )  
 and ni  n = 1.8912(13 )  . from the metrical parameters , 
 the terminal oxygen ( c = o(2 ) = 1.229(2 )  )
is part of a 
 carbonyl functionality , while the  other  half of the 
 co2 has been reduced to a single bond , c 
o(1 ) = 
 1.340(2 )  as part of the metallacycle . when exploring 
 the chemistry of the more electron - rich imido complex 3 , a different outcome
is observed . treating 3 with a 
 bed of co2 resulted in quick formation of the yellow complex 
 ( dtbpe)ni{o , o:(oc(o))2n(1-ad ) } ( 13 ) in 62% isolated yield ( scheme 5 ) .
formation of 13 most likely involves 
 a [ 2 + 2]-cycloaddition intermediate ( dtbpe)ni{o , c : oc(o)n(1-ad ) } , like 12 , followed by co2 insertion into the more electron - rich and less - hindered ni 
the reactivity of co2 with 3 mirrors 
 that of ( dtbpe)ni = cph2 , which resulted in six - membered 
 ring formation observed in ( dtbpe)ni{o , o:(oc(o))2cph2)}. nmr spectra of 13 are indicative of a c2v - symmetric complex , while the ir spectrum 
 shows two carbonyl stretches co = 1667 and 1624 
 cm . attempts to prepare the ( dtbpe)ni{o , c : oc(o)n(1-ad ) } using 1 equiv of co2 resulted in formation of 13 along with unreacted 
 starting material .
for example , 
 electron - rich ti = n bonds tend to undergo metathesis with co2 to form the titanium oxo , while aryl - substituted imido complexes 
 such as cp*ti = n{2,6-me2c6h3}{mec(nipr)2 } can undergo cyloaddition of 1 equiv of co2 to form the n , o - bound carbamate , 
 which can further react with another equivalent of co2 to 
 form the azadicarboxylate ligand [ ( oc(o))2n(2,6-me2c6h3 ) ] .
lastly , we explored the thermal stability 
 of imido 2 because it is known that ( 2,2-bipyridine)niet2 reacts with excess mesitylazide to give azomesitylene ( mesn = nmes ) .
( 2,2-bipyridine)ni = nmes  is a key species en 
 route to the formation of the azomesitylene , a hypothesis that partially 
 prompted the synthesis of 1 .
hence , heating a benzene 
 solution of 3 revealed formation of azomesitylene , which 
 can be isolated as red crystals in 40% yield ( scheme 5 ) .
the thermolysis of 3 demonstrates 
 that the nitrene fragments from a nickel imido can couple to give 
 azoarenes and provides some support to the idea of an imido intermediate 
 in the formal coupling of mesitylazide by ( 2,2-bipyridine)niet2 .
this thermolysis is also related to the report that fe2(co)9 can decompose phenylazide to azobenzene .
in this work we presented 
 three synthetic pathways to mononuclear nickel imido complexes having 
 aliphatic or aromatic groups and reported reactivity involving the 
 ni = n motif .
a reinvestigation utilizing high - level ab initio 
 quantum chemistry techniques is given for the original ni - imido complex 
 reported in 2001 .
the computational analysis of 1 , coupled 
 with the newly disclosed reactivity studies reported herein , paint 
 a picture of this late metal imido complex as being able to effect 
 both one- ( i.e. , radical ) and two - electron ( e.g. , deprotonation of 
 terminal alkynes ) transformations given the biradical and highly nimido - localized nature of the frontier orbitals of 1 . moreover , delocalization of these same frontier orbitals from the 
 / * orbitals of the ninimido active site to 
 the aryl 
imido substituent suggests considerable potential 
 to tune the reactivity of 1 and other late metal imido 
 complexes among these disparate reactivity manifolds through judicious 
 manipulation of the chemical environment about the active site .
in addition to c  h activation reactions ( both homolytically 
 and presumably heterolytically ) , we include examples of cycloaddition 
 chemistry of 1 with various electrophiles including some 
 cumulenes .
taking advantage of the acidic c  h bond in hccph , 
 it is shown that imido group in 1 can couple to acetylide 
 to ultimately form a keteneimine .
the imido moiety can also engage 
 in cycloaddition and subsequent insertion or isomerization pathways 
 to form unusual nickel metallacycles . while 1 seems to 
 only cycloadd small molecules such as ketenes , isocyanates , aldehydes , 
 and co2 , more electron - rich alkyl 
the 
 delocalization of the frontier orbitals between nin and nar / * 
 orbitals is also consistent with the greater reactivity of 3 than 1 .
in contrast , reducing steric bulk on the arylimido 
 group can allow for reductive coupling .
 the 
 three - coordinate imidos ( dtbpe)ni = nr ( dtbpe = tbu2pch2ch2ptbu2 , r = 2,6-ipr2c6h3 , 2,4,6-me3c6h2 ( mes ) , and 1-adamantyl ( ad ) ) , which contain 
 a legitimate ni  
 n double bond as well as basic imido nitrogen 
 based on theoretical analysis , readily deprotonate hccph to 
 form the amide acetylide species ( dtbpe)ni{nh(ar)}(ccph ) . 
 in the case of r 
 = 2,6-ipr2c6h3 , reductive carbonylation results in formation 
 of the ( dtbpe)ni(co)2 along with the n  
 c coupled 
 product keteneimine phch = c = n(2,6- ipr2c6h3 ) . given the ability 
 of the ni = n bond to have biradical character as suggested by 
 theoretical analysis , h atom abstraction can also occur in ( dtbpe)ni = n{2,6-ipr2c6h3 } 
 when this species is treated with hsn(nbu)3 . 
 likewise , the microscopic reverse reaction  conversion 
 of the ni(i ) anilide ( dtbpe)ni{nh(2,6-ipr2c6h3 ) } to the imido ( dtbpe)ni = n{2,6-ipr2c6h3}is 
 promoted when using the radical mes*o ( mes * = 2,4,6-tbu3c6h2 ) . 
 
 reactivity studies involving the imido complexes , in particular ( dtbpe)ni = n{2,6-ipr2c6h3 } , 
 are also reported with small , unsaturated molecules such as diphenylketene , 
 benzylisocyanate , benzaldehyde , and carbon dioxide , including the 
 formation of c  n and n  
 n bonds by coupling reactions . 
 
 in addition to nmr spectroscopic data and combustion analysis , we 
 also report structural studies for all the cycloaddition reactions 
 involving the imido ( dtbpe)ni = n{2,6-ipr2c6h3}. 
Introduction
Experimental Section
Results and Discussion
Conclusions