The eta(2)-acyl-isocyanide complexes Bp’Mo(eta(2)-C(O)R)(CN-t-Bu)CO(PMe(3)) have been prepared from the known acyls Mo(eta(2)-C(O)R)Cl(CO)(PMe(3))(3) by a two-step procedure that involves treatment with the bidentate monoanionic ligand Bp’ (Bp’ = unsubstituted and 3,5-Me(2)-substituted dihydrobis(pyrazolyl)borate, Bp and Bp*, respectively) followed by PMe(3) substitution by CN-t-Bu. These compounds undergo irreversible, thermal isomerization (10-70 degrees C) to the corresponding eta(2)-iminoacyl-carbonyl derivatives Bp’Mo(eta(2)-C(N-t-Bu)R)(CO)(2)(PMe(3)), thereby demonstrating the greater thermodynamic stability of the latter compounds. This isomerization reaction follows first-order kinetics, and it is characterized by activation parameters Delta H-double dagger = 20.3 +/- 1.4 kcal.mol(-1) and Delta S-not subset of = -12.6 +/- 1.2 cal.mol(-1).K-1. A pair of complexes implicated in this acyl-isc,cyanide to iminoacyl-carbonyl isomerization, namely the CH(2)SiMe(3) derivatives Bp*Mo(eta(2)-C(O)CH(2)SiMe(3))(CN-t-Bu)(CO)(PMe(3)) (4b) and Bp*Mo(eta(2)-C(N-t-Bu)CH(2)SiMe(3))(CO)(2)(PMe(3)) (6b), have been structurally characterized by single-crystal X-ray determinations.