An infrared spectroscopic method was devised for measuring delta in 18+delta organometallic complexes containing the chelating diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (L(2)) or 2,3-bis(diphenylphosphino)-2-cyclopentene-1,4-dione (L(2)’). (18+delta complexes are 19-electron complexes in which the unpaired 19th valence electron is primarily localized on a ligand; delta represents the amount of the unpaired electron’s charge on the metal.) delta values for the Co(CO)(3)L(2), Co(CO)(3)L(2)(-), Fe(CO)(3)L(2)’(-), and Fe(CO)(3)L(2)’(-) complexes fall in the range 0.01-0.25. The effects of the metal, the ligands, and the solvent on 6 were quantitatively evaluated. In addition, the effect of delta on the reactivity was examined by studying the dissociative substitution reactions of Co(CO)(3)L(2). The following principles emerged : (1) delta is larger for complexes with a more electronegative metal center (e.g., Co(I) vs Fe(0)). (2) delta is smaller for complexes containing the more electronegative L(2) ligand than for those with the less electronegative L(2)’ (3) delta increases with decreasing solvent polarity, but increases with increasing solvent donicity. (4) For those cases in which delta is manipulated by changing the solvent, there is no simple correlation between delta and the rate of a dissociative substitution reaction in an 18+delta complex. The latter two results are interpreted in terms of a model in which donor solvents increase the electronic population of the pi* SOMO on the L(2) ligand and acceptor solvents decrease the electron density in this orbital. Additional electron density in the pi* orbital increases delocalization of the unpaired electron onto the Co fragment, causing delta to increase and weakening the Co-CO bond. (The acceptor orbital on the Co fragment is Co-CO sigma antibonding.) There is no correlation between the rate constants and 6 because Delta S double dagger effects are significant, especially in polar solvents.