Studies of their i.r. spectra (1700–2100 cm–1) show that solutions of various {LCo(CO)3}2 complexes {L = R3M, R2(R′)M, or p-XC6H4(R)2M (M = P, As, or Sb; R,R′= alkyl; and X = H, Me, MeO, Br, F, and CF3 when R = Et} contain one non-bridged and two CO-bridged isomers. The first are favoured by higher temperatures, polar solvents, bulky ligands, and a higher electron density at the cobalt atom. These factors also affect the relative proportions of the two bridged tautomers which have been identified unequivocally.