the invention tellurium sensitizers have not been previously utilized as chemical sensitizers . the tellurium compounds of the invention have numerous advantages over prior materials . the te ( ii ) coordination complexes of this invention give improved sensitization compared to prior art tellurium sensitizers . the te ( ii ) coordination complexes of this invention exhibit enhanced stability under ambient keeping conditions compared to prior art tellurium sensitizers . the te ( ii ) coordination complexes of this invention can be prepared in high yields by convenient synthetic procedures . tellurium ( ii ) coordination complexes with monodentate and bidentate sulfur ligands have been described in the open literature : ( c ) i . haiduc , r . b . king , and m . g . newton , chem . rev ., 94 , 301 ( 1994 ). any tellurium compound as set forth in formula i or formula ii is a suitable sensitizer . the preferred sensitizers have been found to be the formula ii compounds because of their increased stability under ambient conditions compared to sensitizers of formula i . the most preferred compounds are the following formula ii compounds that provide a dramatic increase in sensitization , are low in cost , and stable : typical compounds of the formula i and formula ii suitable for the invention are listed in table 1 below . the l thiourea or selenourea of formula i may be substituted with 1 - 4 alkyl or aryl substituents on its 2 nitrogen atoms such as : ## str5 ## e = s , se ; r , r &# 39 ;, r &# 34 ;, r &# 34 ;&# 39 ;= alkyl substituents such as methyl , ethyl , propyl , iso - propyl , n - butyl , iso - butyl , n - amyl , iso - amyl , hexyl , cyclohexyl , heptyl , octyl , nonyl , decyl , dodecyl , etc . or aryl such as phenyl , p - tolyl , p - anisyl , pentafluorophenyl , naphthyl . the tellurium complexes of the invention may be utilized in any suitable amount . typically the tellurium compounds would be utilized in an amount between about 0 . 1 and 100 μmol / mol ag . preferably they would be utilized in an amount between about 5 and 50 μmol / mol ag . the tellurium complexes of the invention may be added to a silver halide emulsion at various stages during emulsion preparation and finishing . they may be added during emulsion formation , or they may be added after emulsion formation and after washing of the emulsion . they may be added prior to a heat cycle for chemical sensitization or they may be added during the heat cycle after the emulsion has been brought to an increased temperature . it is preferred that they be added either prior to or during the sensitization cycle . the heat cycle is preferably carried out at a temperature of between about 30 ° and 90 ° c . with a preferred temperature of addition being between 40 ° and 70 ° c . the addition may take place prior to heating or after heating has taken place . the tellurium sensitizing compounds may be added singly or in combination with other sensitizing agents . they also may be added to a silver halide emulsion along with silver ion ligands and silver halide growth modifiers or stabilizers and antifogging agents . further , the tellurium complexes of the invention may be added with other chemical sensitizing agents such as sulfur , selenium , or noble metal compounds such as those of gold , palladium , platinum , rhodium , or iridium compounds or with dopants such as iron , iridium , rhodium , ruthenium , or osmium complexes . they may be added in the presence of spectral sensitizing dyes . the tellurium complexes may be added during formation of silver halide grains , during the physical or chemical ripening stage , or in a separate step immediately prior to coating to form a photographic element . this invention provides a process for chemical sensitization of a silver halide emulsion formed according to any of the processes generally well known in the art . a double jet - type process is preferred . the silver halide grains can comprise mixed or single halide components and especially include chloride , bromide , iodide , iodochloride , iodobromide or chlorobromide grains . they can also be different morphologies such as cubic , octahedra , tabular , or tetradecahedral . the chemical sensitizers of the invention are also suitable for core shell emulsions in which the composition and properties of a silver halide grains core are significantly different than the silver halide composition and properties on the surface of the grains . the double - jet process comprises adding an aqueous silver nitrate solution and an aqueous solution of one or more halides , for example , an alkali metal halide such as potassium bromide , potassium chloride , potassium iodide or mixtures thereof , simultaneously to a stirred solution of a silver halide protective colloid through two separate jets . gelatin is preferred as the binder or protective colloid for the photographic emulsion of the present invention . however , other hydrophilic colloids are also suitable . for example , proteins such as gelatin derivatives , graft polymers of gelatin and other polymers , albumin , casein , cellulose derivatives such as hydroxyethyl cellulose , carboxymethyl cellulose and cellulose sulfate , sugar derivatives such as sodium alginate , starch derivatives and various synthetic peptizers such as hydrophilic homopolymers or copolymers such as polyvinyl alcohol , poly - n - vinylpyrrolidone , polyacrylic acid , polymethacrylic acid , polyacrylamide , polyvinylimidazole and polyvinyl pyrazole can be used . acid - processed gelatin can be used , as well as lime - processed gelatin . further , gelatin hydrolyzates and enzyme - hydrolyzed products of gelatin are also usable . surface - active agents may be incorporated in a photographic emulsion layer or in another hydrophilic colloid layer as a coating aid to prevent buildup of static charge , to improve lubrication properties , to improve emulsion dispersion , to prevent adhesion and to improve other properties . a photosensitive material of the present invention may contain antifogging agents or emulsion - stabilizing agents such as , for example , azaindenes , disulfides , thionamides , azoles and the like . the photographic silver halide emulsions as described can be used in photographic silver halide elements in any of the ways and for purposes known in the photographic art . the photographic silver halide emulsions can be used and incorporated in photographic elements that are black and white , single color elements or multicolor elements . multicolor elements contain dye image - forming units sensitive to each of the three primary regions of the visible spectrum . each unit can be comprised of a single emulsion layer or of multiple emulsion layers sensitive to given regions of the spectrum . the layers of the element can be arranged in various orders as known in the art . in the following discussion of suitable materials for use in emulsions and elements of the invention , reference will be made to research disclosure , number 365 of september 1994 . research disclosure is published by kenneth masons publications ltd ., dudley annex , 12 north street , emsworth , hampshire po 10 7dq , england . this publication will be identified hereafter by the term &# 34 ; research disclosure &# 34 ;. the silver halide emulsions of the invention can be used in elements that can be either negative - working or positive - working . the emulsions in which the described new chemical sensitizers can be used are described in , for example , research disclosure sections i , ii and iii and the publications and patents cited therein . useful vehicles for the emulsion layers and other layers of elements of the invention are described in research disclosure section ix and the publications cited therein . the described photographic emulsions can be used in color photographic elements with couplers as described in research disclosure section x and the publications cited therein . the couplers can be incorporated in the elements and emulsions as described in research disclosure section xi and ways known in the art . the photographic elements and emulsions as described can contain addenda known to be useful in photographic elements and emulsions in the photographic art . the photographic elements and emulsions as described can contain , for example , brighteners ( see research disclosure section vi ); antifoggants and stabilizers ( see research disclosure section vii ); antistain agents and image dye stabilizers ( see research disclosure section x ); light absorbing and scattering materials ( see research disclosure section ii ); hardeners ( see research disclosure section ix ); coating aids ( see research disclosure section ix ); plasticizers and lubricants ( see research disclosure section ix ); antistatic agents ( see research disclosure section ix ); matting agents ( see research disclosure section ix ); and development modifiers ( see research disclosure section xviii ). the photographic silver halide materials and elements as described can be coated on a variety of supports as described in research disclosure section xv and the publications cited therein . the photographic silver halide materials and elements as described can include coarse , regular and fine grain silver halide crystals or mixtures thereof and can be comprised of any photographic silver halides known in the photographic art . the photographic silver halide materials as described can be spectrally sensitized by means and dyes known in the photographic art , such as by means of spectral sensitizing dyes as described in , for example , research disclosure section v and the publications cited therein . combinations of spectral sensitizing dyes are especially useful . photographic materials and elements as described can be exposed to actinic radiation , typically in the visible region of the spectrum , to form a latent image as described in research disclosure section xvi and then processed to form a visible image as described in , for example , research disclosure section xviii using developing agents and other processing agents known in the photographic art . processing to form a visible image , typically a dye image , includes the step of contacting the element with a developing agent , typically a color developing agent , to reduce developable silver halide and oxidize the developing agent . in a color material the oxidized color developing agent in turn reacts with couplers to yield a dye . the photographic silver halide materials can also be used in physical development systems as described in research disclosure section xvii , in image - transfer systems as described in research disclosure section x , in dry development systems as described in research disclosure section xvii and in printing and lithography materials as described in research disclosure section xix . the photosensitive materials obtained by the present invention can be processed according to known methods . a developer to be used for the black - and - white processing can contain conventional developing agents such as dihydroxybenzenes ( e . g ., hydroquinone ), 3 - pyrazolidones ( e . g ., 1 - phenyl - 3 - pyrazolidone ), amino - phenols ( e . g ., n - methyl - p - amino - phenol ), 1 - phenyl - 3 - pyrazolidones or ascorbic acids . as color - developing agent , there can be used primary aromatic amine developing agents such as phenylenediamines ( e . g ., 4 - amino - n , n - diethylaniline , 3 - methyl - 4 - amino - n , n - diethylaniline , 4 - amino - 3 - methyl - n - hydroxyethylaniline , 3 - methyl - 4 - amino - n - ethyl - n - hydroxyethylaniline , 3 - methyl - 4 - amino - n - ethyl - n - methanesulfonamido - ethylaniline and 4 - amino - 3 - methyl - n - ethyl - n - methoxyethylaniline . in addition , the developing agents described in l . f . a . mason , photographic processing chemistry ( focal press , 1966 ), pp . 226 - 229 , as well as those described in u . s . pat . nos . 2 , 193 , 015 and 2 , 592 , 364 may be used . the photographic emulsions of the present invention can be used in many different silver halide photographic light - sensitive materials due to its high photographic sensitivity , contrast , and fog reduction . for example , it can be used in high speed black - and - white negative films , in x - ray films and in multilayer color negative films . the invention is particularly suitable for use with tabular silver bromoiodide grains which find their preferred use in color negative films . in such films it is particularly important that higher speeds be obtained , as there is a continuing need for higher speed films for color negative photography . the following examples illustrate the practice of this invention . they are not intended to be exhaustive of all possible variations of the invention . parts and percentages are by weight unless otherwise indicated . the te ( ii ) coordination complexes of this invention are typically prepared by reacting the sulfur ligand , e . g ., a thiourea type compound or a salt of a 1 , 1 - dithio type ligand as defined above , with an aqueous solution of the te ( 4 +) chloro complex , { tecl 6 }( 2 -), prepared by dissolving tellurium dioxide , teo 2 , in hydrochloric acid or a hydrochloric acid - acetic acid mixture with warming . the stoichiometry of the te ( ii )- thiourea complexes , i . e ., te ( tu ) n x 2 , n = 2 or 4 , is determined by the molar ratio of thiourea to te ( 4 +) used in the reaction . with a ratio of ca 4 : 1 , the bis complexes are obtained : ## str6 ## in these reactions the thiourea is used both as a reducing agent , i . e ., te ( 4 +)→ te ( 2 +), and also as a stabilizing ligand for te ( ii ), preventing its reduction to elemental tellurium . using a 6 : 1 or greater ratio of thiourea / te ( 4 +), the tetrakis complexes , { te ( tu ) 4 } x 2 are formed in these reactions . the molecular complexes with 1 , 1 - dithio type ligands , { te ( s 2 x ) 2 }, can also be prepared by substitution reactions between the labile te ( ii ) complex , na 2 { te ( s 2 o 3 ) 2 }. 2h 2 o , and two equivalents of a salt of the 1 , 1 - dithio ligands -- typically sodium or potassium salts are used . these reactions are carried out in aqueous solution with previously isolated na 2 { te ( s 2 o 3 ) 2 }. 2h 2 o , prepared as described in o . foss , inorganic synthesis , 4 , 89 ( 1953 ), or with this complex generated in - situ by the reduction of { tecl 6 }( 2 -) with excess sodium thiosulfate in an ice bath as described in examples below . the molecular complexes precipitate from the reaction solutions and , after isolation by filtration , washing with water and drying , they can be purified by recrystallization from organic solvents . the thermal stabilities of these complexes vary , depending on the nature of the bidentate sulfur ligand used but , in general , they exhibit enhanced stability compared to the above te ( ii ) complexes with monodentate thiourea ligands . thus , by proper choice of te ( ii ) complex , as well as the length and temperature of the emulsion finishing step , the rate of introduction of tellurium sensitization centers can be readily controlled using the te ( ii ) complexes of this invention . tellurium dioxide ( 3 . 2 g , 20 mmoles ) was dissolved , with warming , in 50 ml . of conc . hcl and 50 ml of water at 85 ° c . to the resulting yellow solution was added a solution of thiourea ( 6 . 1 g , 80 mmoles ) dissolved in 75 ml of warm water . the yellow solution immediately turned red , and on cooling the reaction solution to 30 ° c . a crop of bright yellow crystals deposited . after further stirring for 10 min ., the solid was isolated by filtration and vacuum dried { 6 . 6 g ( 94 % yield ): calcd . ( found ) for c 2 h 8 n 4 s 2 cl 2 te ( m . w .= 350 . 75 ): c , 6 . 85 ( 6 . 71 ); h , 2 . 30 ( 2 . 22 ); n , 15 . 97 ( 15 . 93 ); s , 18 . 28 ) ( 18 . 20 )}. teo 2 ( 4 . 8 g , 30 mmoles ) was dissolved with warming in a solution of 20 ml concentrated hydrochloric acid and 20 ml of water . to the resulting yellow solution ( 85 ° c .) was added a solution of thiourea ( 15 g , 198 moles ) dissolved in 100 ml of warm water . cooling the resulting red reaction solution to 30 ° c . gave a crop of yellow crystals which were isolated by filtration and vacuum dried { yield = 15 . 2 g ( 94 %): calcd . ( found c 4 h 20 n 8 o 2 s 4 cl 2 te ( f . w .= 539 . 02 ): c , 8 . 91 ( 8 . 75 ); h , 3 . 74 ( 3 . 5 ); n , 20 . 79 ( 20 . 36 ); s , 23 . 80 ( 23 . 5 )} tellurium dioxide ( 3 . 2 g , 20 moles ) was dissolved , with warming , in 50 ml . of conc . hbr and 50 ml of water at 85 ° c . to the resulting yellow solution was added a solution of thiourea ( 6 . 1 g , 80 moles ) dissolved in 75 ml of warm water . the yellow solution immediately turned red and on further stirring for ca . 1 min . a heavy orange precipitate began to deposit . the reaction solution was further stirred for 10 min . and the orange solid was isolated by filtration and vacuum dried { 8 . 35 g ( 95 %): calcd . ( found ) for c 2 h 8 n 4 s 2 br 2 te ( m . w .= 439 . 65 ): c , 5 . 46 ( 5 . 2 ); h , 1 . 83 ( 1 . 9 ); n , 12 . 75 ( 12 . 5 ); s , 14 . 59 ( 14 . 0 ); te , 29 . 02 ( 30 . 3 )}. this product is moderately soluble in n , n - dimethylformamide and methanol . concentration of the above aqueous filtrate gave a crop of the thiourea oxidation product as yellow crystals , i . e ., the formamidinium disulfide salt {( h2n ) 2 c - s - s - c ( nh 2 ) 2 } br 2 . to a solution of 17 . 3 g ( 40 moles ) of na 2 { te ( s 2 o 3 ) 2 }. 2h 2 o , prepared as described in o . foss , inorganic synthesis , 4 , 89 ( 1953 ) dissolved in 750 ml of water at - 5 ° c . was slowly added , from a dropping funnel , a solution of 17 . 4 g ( 100 moles ) of ks 2 co - i - c 3 h 7 ( prepared by reacting cs 2 with an isopropanol solution of potassium hydroxide as described in l . c . blackman and m . j . dewar , j . chem . soc ., 162 ( 1957 )) dissolved in 200 ml of water . after addition of all the potassium isopropyl xanthate solution , the reaction solution was stirred a further 15 minutes , and the brown solid which had precipitated was isolated by filtration , washed well with water , and air dried to give 13 . 8 grams of crude product . the crude product was recrystallized from 600 ml of 2 : 1 i - propanol - benzene to give 10 . 7 g of red needles ( 67 %). this complex , bis ( iso - propylxanthato ) tellurium ( ii ), is soluble in n , n - dimethylformamide , acetone , methylene chloride , toluene and methanol , but insoluble in water . calcd . ( found ) for c 8 h 14 o 2 s 4 te ( mw = 398 . 06 ): c , 24 . 14 ( 24 . 17 ); h , 3 . 55 ( 3 . 35 ); s , 32 . 22 ( 32 . 36 ) this example illustrates the synthesis of te ( ii ) complexes with 1 , 1 - dithio type ligands using in - situ generated { te ( s 2 o 3 ) 2 }( 2 -). tellurium dioxide ( 1 . 6 g , 10 mmoles ) was dissolved with warming in a mixture of 4 ml of conc . hcl and 7 ml of glacial acetic acid . after complete dissolution of the teo 2 , the solution was cooled to - 10 ° c . in an ice - salt bath and a solution of 10 g of sodium thiosulfate pentahydrate ( na 2 s 2 o 3 . 5 h 2 o ) in 5 ml of water was added dropwise to the above solution ( maintained at - 10 ° c . in the cold bath ). after addition of all the sodium thiosulfate solution , an additional 25 ml of the hydrochloric acid - glacial acetic acid mixture ( at - 10 ° c .) was added . to the resulting solution ( kept in the - 10 ° c . cold bath ) a solution of ks 2 coet ( 4 . 8 g , 30 mmoles dissolved in 150 ml of water ) was added dropwise from a separatory funnel . after complete addition of the potassium ethyl xanthate solution , the reaction solution ( containing a red - brown precipitate ) was diluted to 1 l with water and stirred another 15 minutes at room temperature , and the solid was isolated by filtration and washed well with water . the isolated solid was vacuum dried and then recrystallized from 150 ml of 1 : 1 toluene - ethanol ( 65 ° c .) to give 1 . 9 g ( 51 %) of the complex as red needles ( calcd . ( found ) for c 6 h 10 o 2 s 4 te ( m . w .= 370 . 01 ): c , 19 . 48 ( 19 . 3 ); h , 2 . 72 ( 2 . 5 ); s , 34 . 67 ( 34 . 9 ); te , 34 . 49 ( 34 . 2 )). tellurium dioxide ( 1 . 6 g , 10 mmoles ) was dissolved with warming in a mixture of 4 ml of conc . hcl and 7 ml of glacial acetic acid with warming . after complete dissolution of the teo 2 , the solution was cooled to - 10 ° c . in an ice - salt bath , and a solution of 10 g of sodium thiosulfate pentahydrate ( na 2 s 2 o 3 . 5 h 2 o ) in 5 ml of water was added dropwise to the above solution ( maintained at - 10 ° c . in the cold bath ). after addition of all the sodium thiosulfate solution , an additional 25 ml of the hydrochloric acid - glacial acetic acid mixture ( at - 10 ° c .) was added . to the resulting solution ( kept in the - 10 ° c . cold bath ) a solution of ks 2 co - i - c 4 h 9 ( 5 . 6 g , 30 mmoles dissolved in 125 ml of water ) was added dropwise from a separatory funnel . after complete addition of the potassium i - butyl xanthate solution , the reaction solution ( containing a red precipitate ) was diluted to 1 l with water and stirred another 15 minutes at room temperature , and the solid was isolated by filtration and washed well with water . the isolated solid was vacuum dried and then recrystallized from 100 ml of i - butanol ( 80 ° c .) to give 2 . 7 g ( 63 . 4 % yield ) of the complex as red needles ( calcd . ( found ) c 10 h 18 o 2 s 4 te ( m . w .= 426 . 12 ): c , 28 . 19 ( 28 . 0 ); h , 4 . 26 ( 4 . 3 ); s , 30 . 10 ( 30 . 0 ); te , 29 . 95 ( 30 . 3 )). tellurium dioxide ( 1 . 6 g , 10 mmoles ) was dissolved with warming in a mixture of 4 ml of conc . hcl and 7 ml of glacial acetic acid with warming . after complete dissolution of the teo 2 , the solution was cooled to - 10 ° c . in an ice - salt bath , and a solution of 10 g of sodium thiosulfate pentahydrate ( na 2 s 2 o 3 . 5 h 2 o ) in 5 ml of water was added dropwise to the above solution ( maintained at - 10 ° c . in the cold bath ). after addition of all the sodium thiosulfate solution , an additional 25 ml of the hydrochloric acid - glacial acetic acid mixture ( at - 10 ° c .) was added . to the resulting solution ( kept in the - 10 ° c . cold bath ) a solution of ks 2 co - n - c 5 h 11 ( 6 . 5 g , 30 mmoles ) dissolved in 150 ml of water ) was added dropwise from a separatory funnel . after complete addition of the potassium amyl xanthate solution , the reaction solution , containing an orange - brown precipitate , was stirred for 15 min . at room temperature and then diluted to 1 l with water , stirred another 5 min ., and the precipitate was isolated by filtration , washed well with water , and vacuum dried to give 4 . 2 g of an orange - brown powder ( 92 . 5 % yield ). this crude product was then recrystallized from 225 ml of 4 : 1 n - butanol - benzene ( 60 ° c .) to give a crop of orange needles ( 2 . 1 g ; 46 . 3 % yield ; calcd . ( found ) c 12 h 22 o 2 s 4 te ( m . w .= 454 . 16 ): c , 31 . 37 ( 31 . 5 ); h , 4 . 88 ( 4 . 4 ); s , 28 . 24 ( 28 . 5 ); te , 28 . 10 ( 28 . 1 )). to a solution of 8 . 7 g ( 20 mmoles ) of na 2 { te ( s 2 o 3 ) 2 }. 2h 2 o , prepared as described in o . foss , inorganic synthesis , 4 , 89 ( 1953 ), dissolved in 500 ml of water at - 5 ° c . was slowly added , from a dropping funnel , a solution of 15 g ( 50 mmoles ) of ks 2 co - n - c 12 h 25 , prepared by reacting cs 2 with a n - dodecanol solution of potassium hydroxide as described in l . c . blackman and m . j . dewar , j . chem . soc ., 162 ( 1957 ), dissolved in 500 ml of methanol . after addition of all the potassium n - dodecyl xanthate solution , the reaction solution , containing a heavy brown precipitate , was further stirred for 30 min . at room temperature , and the precipitate was isolated by filtration , washed well with water and methanol , and vacuum dried to give 11 . 9 g of a brown powder ( 91 . 5 % yield ). this crude product was recrystallized from 350 ml of 2 : 1 isopropanol - benzene ( 75 ° c .). filtration of this hot solution through a medium porosity glass frit , followed by cooling overnight at - 10 ° c . gave a crop of an orange fibrous solid ( 8 . 2 g ( 63 %); calcd . ( found ) for c 26 50 o 2 s 4 te ( mw = 650 . 52 ): c , 48 . 0 ( 47 . 6 ); h , 7 . 75 ( 8 . 3 ); s , 19 . 73 ( 20 . 1 )). to a suspension of { te ( thiourea ) 4 } cl 2 . 2h 2 o ( 5 . 03 g , 10 mm ), prepared as in example 2 , in 50 ml of methanol was added a methanol solution of 51 mm of hs 2 cn ( ch 2 ch 2 oh ) 2 , prepared in situ by reacting hn ( ch 2 ch 2 oh ) 2 ( 51 mm ; 3 . 0 ml ) and 4 . 9 g of cs 2 in 40 ml of methanol - water ( 4 : 1 ). after stirring the reaction solution for 1 hr , it was diluted to 400 ml with water to give a red oil . the aqueous solution was decanted from the red oil and the oil was washed with two - 100 ml portions of water . the red oil was then recrystallized from 150 ml of hot methanol . after filtering the methanol solution , concentrating the filtrate to 50 ml and cooling overnight in a refrigerator , a peach solid deposited which was isolated by filtrate , washed with 30 ml of cold meoh and vacuum dried ( yield = 3 . 2 g ; calcd . ( found ) for c 10 h 20 n 2 o 4 s 4 te , m . w .= 488 . 14 : c , 24 . 61 ( 24 . 4 ); h , 4 . 13 ( 3 . 88 ); n , 5 . 74 ( 5 . 62 ); s , 26 . 28 ( 7 . 08 )). synthesis of te { s 2 cn ( n - c 4 h 9 ) 2 } 2 tellurium dioxide ( 1 . 6 g , 10 mmoles ) was dissolved in a mixture of 4 ml of conc . hcl and 7 ml of glacial acetic acid with warming . after complete dissolution of the teo 2 , the solution was cooled to - 10 ° c . in an ice - salt bath , and a solution of 10 g of sodium thiosulfate pentahydrate ( na 2 s 2 o 3 . 5 h 2 o ) in 5 ml of water was added dropwise to the above solution ( maintained at - 10 ° c . in the cold bath ). after addition of all the sodium thiosulfate solution , an additional 25 ml of the hydrochloric acid - glacial acetic acid mixture at - 10 ° c . was added . to the resulting solution ( kept in the - 10 ° c . cold bath ) a solution of nas 2 cn ( n - bu ) 2 ( 6 . 82 g , 30 mmoles ) dissolved in 150 ml of water was added dropwise from a separatory funnel . after complete addition of the sodium di ( n - butyl ) dithiocarbamate solution , the reaction solution , containing a red - brown precipitate , was stirred for 15 min . at room temperature and then diluted to 1 l with water , stirred another 5 min . and the precipitate was isolated by filtration , washed well with water , and vacuum dried to give 5 . 1 g of a red - brown powder ( 95 % yield ). this crude product was then recrystallized from 250 ml of 1 : 1 ethanol - benzene ( 60 ° c .) to give a crop of dark red crystals ( 3 . 9 g ; 72 . 8 % yield ; calcd . ( found ) c 18 h 36 n 2 s 4 te ( m . w .= 536 . 36 ): c , 40 . 31 ( 40 . 5 ); h , 6 . 77 ( 6 . 4 ); s , 23 . 91 ( 23 . 5 )). teo 2 ( 4 . 8 g , 30 mm ) was dissolved in 25 ml of concentrated hcl with warming . the resulting clear yellow solution was diluted with 75 ml of water and cooled to - 10 ° c . in an ice - salt bath . to this cold solution was added , dropwise , a solution of nh 4 { s 2 p ( oet ) 2 } ( 24 . 4g , 120 mm ) in 200 ml of water . the reaction solution was allowed to warm to room temperature and stirred for 1 hr . the aqueous solution was then decanted from the gummy red product , and it was further washed with 2 - 100 ml portions of water . trituration of the gummy red product with 200 ml of ethanol gave an orange powder which was isolated by filtration and air dried ( yield : 11 . 4 g ). this product was recrystallized from 250 ml of 60 ° c . ethanol . cooling the bright red filtrate overnight in a refrigerator gave a crop of deep red crystals 5 . 2 g ) ( calcd . ( found ) for c 8 h 20 o 4 p 2 s 4 te ( m . w .= 498 . 06 ): c , 19 . 29 ( 19 . 37 ); h , 4 . 05 ( 4 . 10 ); s , 25 . 75 ( 25 . 62 ); p = 12 . 44 ( 12 . 12 )) . a monodisperse agbr tabular emulsion , prepared as taught in u . s . pat . no . 5 , 147 , 771 , with an equivalent circular diameter of 2 . 4 μm and a thickness of 0 . 138 μm was treated with the tellurium compounds to sensitize at a variety of levels and temperatures for 20 minutes as shown in table 2 . once the chemical digestion was complete the example emulsions were cooled and coated on a film support at 1614 mg ag m - 2 and 3230 mg gel m - 2 . a 1614 mg gel m - 2 overcoat was applied over the emulsion containing layers . the coatings were then dried and exposed ( 0 . 1 s , 365 nm source ) through a graduated density step wedge , processed ( 6 minutes at 20 ° c .) in kodak rapid x - ray developer , washed , and dried . speeds are expressed as the relative exposure required to increase the measured density to 0 . 15 above fog . table 2______________________________________sensitization of the agbr tabular emulsion by te compounds amount added relativeformula ( μmole ) temperature speed______________________________________ -- -- -- 100 ( a ) na . sub . 2 s . sub . 2 o . sub . 3 25 60 380 ( b ) te ═ p ( i - pr ). sub . 3 10 40 48 ( c ) te ( p - anisyl ). sub . 2 25 40 & amp ; 60 100te ( thiourea ). sub . 4 cl . sub . 2 25 60 646te ( s . sub . 2 coet ). sub . 2 10 40 251te ( s . sub . 2 co - i - pr ). sub . 2 25 60 603te ( s . sub . 2 co - i - bu ). sub . 2 10 60 676te ( s . sub . 2 coc . sub . 12 h . sub . 25 ). sub . 2 10 60 724te ( s . sub . 2 nh ( ch . sub . 2 ch . sub . 2 oh ). sub . 2 ). sub . 2 25 40 646______________________________________ ( a ) comparative example ( b ) comparative example with compound described in u . s . pat . no . 5 , 215 , 880 ( 1993 ) ( c ) comparative example with compound described in japanese patent application no . 57817 / 53 ( 1978 ) the invention has been described in detail with particular reference to preferred embodiments thereof , but it will be understood that variations and modifications can be effected within the spirit and scope of the invention .