Reaction of t-butylisocyanide with Co(ClO 4) 2· 6H 2O in 4:1 molar ratio in EtOH at O °C produces [Co(CNCMe 3) 4H 2O](ClO 4) 2 in near quantitative yields. Ligand substitution/reduction reactions of [Co(CNCMe 3) 4H 2O] (ClO 4) 2 (in CH 3CN) with triarylphosphines (in CH 2Cl 2) at 25 °C are rapid and in good yield;[Co(CNCMe 3) 3{P(C 6H 4OMe- p) 3} 2]ClO 4, [Co(CNCMe 3) 3{P(C 6H 4Cl- p) 3} 2]ClO 4, and [Co(CNCMe 3) 3{P(C 6H 5) 3} 2]ClO 4 were prepared. Analogous reaction with As(C 6H 5) 3 caused intense color change, but only unsubstituted [Co(CNCMe 3) 4H 2O](ClO 4) 2 was recovered. Reaction of [Co(CNCMe 3) 4H 2O](ClO 4) 2 with trialkylphosphines produced [Co(CNCMe 3) 3{P(C 6H 13-n) 3} 2] ClO 4, [Co(CNCMe 3) 3{P(CH 2CH 2CN) 3}2] ClO 4, and [Co(CNCMe 3) 3{P(NMe 2) 3} 2]ClO 4, but anticipated monosubstituted product with P(NEt 2) 3 was not recovered. Infrared, 1H NMR and electronic spectra give some indication of coordination structure.