MEASUREMENT AND MANIPULATION OF THE UNPAIRED ELECTRON-DENSITY IN 18- CORRELATION OF THE CHARGE-DENSITY WITH REACTIVITY(DELTA COMPLEXES )

An infrared spectroscopic method was devised for measuring delta in 18 +delta organometallic complexes containing the chelating diphosphine l igand 2,3-bis(diphenylphosphino)maleic anhydride (L(2)) or ,3-bis(diph enylphosphino)-2-cyclopentene-1,4-dione (L(2)'). (18+delta complexes a re 19-electron complexes in which the unpaired 19th valence electron i s 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)'(-) com plexes fall in the range 0.01-0.25. The effects of the metal, the liga nds, and the solvent on 6 were quantitatively evaluated. In addition, the effect of delta on the reactivity was examined by studying the dis sociative substitution reactions of Co(CO)(3)L(2). The following princ iples emerged: (1) delta is larger for complexes with a more electrone gative metal center (e.g., Co(I) vs Fe(0)). (2) delta is smaller for c omplexes containing the more electronegative L(2) ligand than for thos e with the less electronegative L(2)' (3) delta increases with decreas ing solvent polarity, but increases with increasing solvent donicity. (4) For those cases in which delta is manipulated by changing the solv ent, there is no simple correlation between delta and the rate of a di ssociative substitution reaction in an 18+delta complex. The latter tw o 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. A dditional electron density in the pi orbital increases delocalization of the unpaired electron onto the Co fragment, causing delta to incre ase and weakening the Co-CO bond. (The acceptor orbital on the Co frag ment is Co-CO sigma antibonding.) There is no correlation between the rate constants and 6 because Delta S double dagger effects are signifi cant, especially in polar solvents.