WHY DO CATIONIC CARBON-MONOXIDE COMPLEXES HAVE HIGH C-O STRETCHING FORCE-CONSTANTS AND SHORT C-O BONDS - ELECTROSTATIC EFFECTS, NOT SIGMA-BONDING

A significant increase in the C-O stretching force constant (F-CO) and a decrease in C-O bond length (r(CO)) result upon coordination of car bon monoxide to various cationic species. We report a study designed t o elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5 sigma orbital of CO, an orbital generally considered to have some antibonding charac ter. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond ax is) reveal that a simple Coulombic field increases the C-O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3+ is calculated to increas e F-CO (and decrease r(CO)) to extents slightly less than those engend ered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5 sigma orbital has no intrinsic positive effect on the magnitude of F-CO. Calculatio ns were also conducted on several symmetrical, neutral, and cationic t ransition metal complexes, including some examples of the recently dis covered homoleptic noble-metal carbonyls. it is found that F-CO values can be quantitatively interpreted using a model which invokes only th e effects of M-CO pi-back-bonding and an electrostatic parameter. Ther e is no correlation between the extent of sigma-bonding (as measured b y the depopulation of the CO sigma orbitals) and F-CO. Calculations on trigonal bipyramidal d(8) metal pentacarbonyls permit a comparison be tween inequivalent ligands (axial and equatorial) which, being coordin ated to the same metal center, must experience approximately the same electrostatic field. In the case of Ru(CO)(5), pi-back-bonding to the axial and equatorial carbonyls is of virtually equal magnitude, while sigma-donation is much greater from the axial ligands than from the eq uatorial ligands. Nevertheless, the F-CO and r(CO) values of the two l igand sets are essentially equal, confirming that the magnitude of sig ma-donation does not affect these parameters.