Transition-Metal Complexes of Tetrylones [(CO)5W-(PPh3)2] and Tetrylenes [(CO)5W-NHE] (E=C-Pb): A Theoretical Study

Quantum chemical calculations at the BP86/TZVPP//BP86/SVP level are performed for the tetrylone complexes [W(CO)(5)-E(PPh3)(2)] (W-1E) and the tetrylene complexes [W(CO)(5)-NHE] (W-2E) with E=C-Pb. The bonding is analyzed using charge and energy decomposition methods. The carbone ligand C(PPh3) is bonded head-on to the metal in W-1C, but the tetrylone ligands E(PPh3)(2) are bonded side-on in the heavier homologues W-1Si to W-1Pb. The W-E bond dissociation energies (BDEs) increase from the lighter to the heavier homologues (W-1C: D-e=25.1 kcal mol(-1); W-1Pb: D-e=44.6 kcal mol(-1)). The W(CO)(5)<- C-(PPh3)(2) donation in W-1C comes from the sigma lone-pair orbital of C(PPh3) 2, whereas the W(CO)(5)<- E(PPh3)(2) donation in the side-on bonded complexes with E=Si-Pb arises from the pi lone-pair orbital of E(PPh3)(2) (the HOMO of the free ligand). The pi-HOMO energy level rises continuously for the heavier homologues, and the hybridization has greater p character, making the heavier tetrylones stronger donors than the lighter systems, because tetrylones have two lone-pair orbitals available for donation. Energy decomposition analysis (EDA) in conjunction with natural orbital for chemical valence (NOCV) suggests that the W-E BDE trend in W-1E comes from the increase in W(CO)(5)<- E(PPh3)(2) donation and from stronger electrostatic attraction, and that the E(PPh3)(2) ligands are strong sigma-donors and weak pi-donors. The NHE ligands in the W-2E complexes are bonded end-on for E=C, Si, and Ge, but side-on for E=Sn and Pb. The W-E BDE trend is opposite to that of the W-1E complexes. The NHE ligands are strong sigma-donors and weak pi-acceptors. The observed trend arises because the hybridization of the donor orbital at atom E in W-2E has much greater s character than that in W-1E, and even increases for heavier atoms, because the tetrylenes have only one lone-pair orbital available for donation. In addition, the W-E bonds of the heavier systems W-2E are strongly polarized toward atom E, so the electrostatic attraction with the tungsten atom is weak. The BDEs calculated for the W-E bonds in W-1E, W-2E and the less bulky tetrylone complexes [W(CO)(5)-E(PH3)(2)] (W-3E) show that the effect of bulky ligands may obscure the intrinsic W-E bond strength.