Organic nitroso complexes can be prepared from preformed organic nitroso precursors.  These precursors usually exist as N-N bonded dimers, but the dimer dissociates readily. This direct method is used to give W(CO)(tert-BuNO) (where tert-Bu is ).  The Fe-porphyrin complex depicted below is prepared by this route. More complicated but more biorelevant routes involve degradation of precursors such as nitrobenzene and phenylhydroxylamine.  
:  (Et = CH, i-Pr = (CH)CH)
The coupling of organic ligands and nitric oxide is yet another route.