Photoelectron spectra of substituted chromium, molybdenum, and tungsten pentacarbonyls. Relative .pi.-acceptor and .sigma.-donor properties of various phosphorus ligands
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The UV photoelectron spectra (PES) of the transition-metal complexes LM(CO)5, where M=Cr, Mo, or W and L=PEt3, PMe3, P(NMe2)3, P(OEt)3, P(OMe)3, or PF3, are reported and compared with the PES of the uncoordinated ligands. Particular emphasis is placed on the assignment of the metal d orbital band components, the M-P bond, and the P nonbonding pair in the free ligand. The ability of the ligands to split the t2g orbitals of the parent hexacarbonyl into the e and b2 components falls in the order PEt3~PMe3>P(NMe2)3>P(OEt)3~P(OMe)3>PF3 and follows the inverted order of π-acceptor ability of these ligands. The σ-donor ability is reflected in the ionization potential (IP) of the M-P bond and falls in the order PEt3~PMe3>P(OEt)3~P(OMe)3>P(NMe2)3>PF3. Comparisons of the free and complexed ligands allow us to make definitive assignments of the PES bands of the free ligands, assignments about which there has recently been considerable controversy. The spin-orbit coupling (SOC) parameters of the W complexes remain unexpectedly constant through this series of ligands rather than decreasing as their π-acceptor ability increases. We attribute this constancy to the ability of CO to release electron density to the metal, thus, compensating for loss of density as the π-acceptor ability of L increases. Therefore, the total delocalization of the metal remains constant. The σ/πparameters derived from the PES are also compared with those from CO force constants. Fenske-Hall molecular orbital (MO) calculations were done on the Cr complexes of PMe3, P(NMe2)3, P(OMe)3, and PF3 and the results of these calculations support our assignments. © 1978, American Chemical Society. All rights reserved.
author list (cited authors)
Yarbrough, L. W., & Hall, M. B.