PHOTOIONIZATION STUDIES OF METAL CARBONYLS .I. IONIZATION POTENTIALS AND BONDING IN GROUP VI METAL HEXACARBONYLS AND IN MONONUCLEAR CARBONYLS AND NITROSYL CARBONYLS OF IRON, COBALT, AND NICKEL

Adiabatic and vertical ionization potentials (IP) have been measured, using a photoionization spectrometer, for the hexacarbonyls of chromium, molybdenum, and tungsten and for iron pentacarbonyl, nickel tetracarbonyl, cobalt nitrosyl tricarbonyl, and iron dinitrosyl dicarbonyl, and upper limits to the bond energies in the molecular ions have been deduced. Photoelectron spectral data are also presented for iron pentacarbonyl and nickel tetracarbonyl. Previous ionization values for these molecules are compared with the new values, and the data are discussed in terms of simple molecular orbital schemes for the uppermost occupied orbitals. It is shown that the results may be interpreted in terms of strong π “back-donation” from metal to ligand, which for the hexacarbonyls increases in the sequence Mo(CO)6 < Cr(CO)6 < W(CO)6. Correlations with other bonding data are discussed and a rationalization of this sequence is suggested. It is also proposed that the charges on the metal atoms in all of these compounds are probably close to zero. © 1969, American Chemical Society. All rights reserved.