THEORETICAL INVESTIGATION OF THE PI-BONDING ABILITY OF P-CONTAINING, S-CONTAINING, AND N-CONTAINING LIGANDS IN GROUP-6 TRANSITION-METAL COMPLEXES

The pi-bonding ability of several ligands bonding through N, P, and S to group 6 transition metals was studied via non-empirical molecular o rbital calculations. The ability of the ligands to donate or accept pi electrons to or from the metal was compared in 1,2-Mo-2[P(t-Bu)(2)](2 )NMe2](4) [I] and 1,2-W-2[P( t-Bu)(2)](2)[NMe2](4) [II]; (PSO)Cr(CO)(4 ) [III] and (PSO)W(CO)(4) [IV] (PSO = 2-diphenyl phosphino methyl-2-ph enylthiomethyl-1-methoxy propane); and [W(2-Spy)(CO)(4)](-) [V] and [M o(2-Spy)(CO)(4)](-) [VI] (2-Spy=2-thiol pyridine). Mulliken population analyses were used to study how two different atoms compete for pi-bo nding with the metal. Both NMe2 and PMe2 are pi donors to the metal in complexes [I] and [II]. Overall, the phosphido ligand is a better pi donor than the amido ligand. The PSO ligand in complexes [III] and [IV ] is a good pi acceptor and phosphorus is a better pi acceptor than su lfur in both complexes. The 2-Spy ligand in the two anionic complexes, [V] and [VI], is a good pi acceptor. pi back-donation from the metal d orbitals to the ligand is mostly through the nitrogen of the pyridin e ring rather than the sulfur external to the pyridine ring. The elect ron density withdrawn from the metal through the nitrogen is delocaliz ed throughout the pi-conjugated ring. The pi-accepting ability of the 2-Spy ligand in the two complexes is about the same. The observed diff erences in the pi-bonding ability of the ligands are explained in term s of the accessibility of the ligand molecular oribitals and the geome try of the complexes. A comparison of pi-bonding between the two linki ng atoms in the same bidentate ligand is also made in terms of the typ e and energy of ligand orbitals (compound [III] vs [IV] and [V] vs [VI ]). A direct comparison is made between electron density distribution and crystallographic bond lengths. (C) 1998 Elsevier Science Ltd. All rights reserved.