The hydride binding energies of Fe(CO) 5, Cr(CO) 6, Mo(CO) 6 and W(CO) 6 ( HA(M(CO) n ) = D[(CO) n− MCOH −]) have been estimated from bimolecular hydride transfer reactions in the gas phase. Hydride transfer to the metal carbonyls yields the corresponding anionic formyl complexes and the hydride affinity bracketing experiments establish HA(Fe(CO) 5) = 56±4 kcal mol −1 and HA(Cr(CO) 6) = HA(Mo(CO) 6) = HA(W(CO) 6) = 44±4 kcal mol −1. Gas-phase heats of formation and metal-carbon bond strengths for the metal formyl anions are derived from the hydride binding energies. The hydride affinity ordering for the metal carbonyls parallels their relative reactivities towards nucleophilic addition in solution and correlates with the relative magnitudes of their CO-stretching force constants ( k co). The new metal formyl data are combined with known or estimated heats of formation for metal hydrides to show that CO-insertion into the metal-hydrogen bond is thermodynamically unfavourable for these systems.