Relationships between bond energies in coordinatively unsaturated and coordinatively saturated transition-metal complexes: A quantitative guide for single, double, and triple bonds

A prescription is presented for converting M⁺-X bond energies (from experiment or theory) in unsaturated complexes to M⁺-X bond energies appropriate for coordinatively saturated organometallic compounds. The theoretical basis for the predicted conversion factors originates from quantitatively evaluating the consequences of (i) the loss of high-spin coupling (exchange energy) between valence electrons on the unsaturated transition-metal ion subsequent to the formation of covalent metal-ligand bonds, (ii) the cost (promotional energy) of bonding to a low-lying excited state of the metal ion (either s¹d^n-1 or dⁿ) instead of to the ground electronic state, and (iii) the loss of high-spin coupling in coordinatively saturated transition-metal complexes upon bond formation (assuming a dⁿ valence electron configuration). These predictions should be most useful for covalent metal-ligand bonds in complexes where the metal has at least a +1 oxidation state and where the ligands of interest have electronegativities comparable to carbon or hydrogen. This method is not appropriate for prediction of bond strengths where the bonds are primarily of ionic or donor-acceptor character.