Ab initio molecular orbital calculations have been used to study the isocyanide-cyanide rearrangement involving the migration of the hydrogen, amino and phosphino groups. Geometries were optimized using MP2/6-311 G (d,p) while relative energies were obtained from CISDQ/6-311 + + G (d,p) wavefunctions. Structures and some spectroscopic properties of the equilibrium structures are predicted. Phosphinoisocyanide is found to lie 16 +/- 2 kcal mol(-1) above phosphinocyanide, and to rearrange to the latter with an energy barrier of 27 +/- 2 kcal mol(-1). Energetics (kcal mol(-1)) of two other rearrangements are as follows: isocyanamide (0) --> TS (37) --> cyanamide (-44) and hydrogen isocyanide (0) --> TS (30) --> hydrogen cyanide (-14). Phosphinoisocyanide exhibits kinetic and thermodynamic stability similar to hydrogen isocyanide (relative to their isomers) and should thus be detectable in both gaseous and condensed phases.