Theoretical investigation of the pi-bonding ability of P-, S-, and N-containing ligands in group 6 transition metal complexes

摘要

The pi-bonding ability of several ligands bonding through N, P, and S to group 6 transition metals was studied via non-empirical molecular orbital 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-phenylthiomethyl-1-methoxy propane); and [W(2-Spy)(CO)(4)](-) [V] and [Mo(2-Spy)(CO)(4)](-) [VI] (2-Spy=2-thiol pyridine). Mulliken population analyses were used to study how two different atoms compete for pi-bonding 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 sulfur 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 pyridine ring rather than the sulfur external to the pyridine ring. The electron density withdrawn from the metal through the nitrogen is delocalized throughout the pi-conjugated ring. The pi-accepting ability of the 2-Spy ligand in the two complexes is about the same. The observed differences in the pi-bonding ability of the ligands are explained in terms of the accessibility of the ligand molecular oribitals and the geometry of the complexes. A comparison of pi-bonding between the two linking atoms in the same bidentate ligand is also made in terms of the type 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. [References: 20]