Transition-Metal Complexes of Tetrylones [(CO)5W-E(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‐1 E) and the tetrylene complexes [W(CO)5‐NHE] (W‐2 E) 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‐1 C, but the tetrylone ligands E(PPh3)2 are bonded side‐on in the heavier homologues W‐1 Si to W‐1 Pb. The WE bond dissociation energies (BDEs) increase from the lighter to the heavier homologues (W‐1 C: De=25.1 kcal mol−1; W‐1 Pb: De=44.6 kcal mol−1). The W(CO)5←C(PPh3)2 donation in W‐1 C comes from the σ 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 π lone‐pair orbital of E(PPh3)2 (the HOMO of the free ligand). The π‐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‐1 E 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 σ‐donors and weak π‐donors. The NHE ligands in the W‐2 E 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‐1 E complexes. The NHE ligands are strong σ‐donors and weak π‐acceptors. The observed trend arises because the hybridization of the donor orbital at atom E in W‐2 E has much greater s character than that in W‐1 E, 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‐2 E 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‐1 E, W‐2 E and the less bulky tetrylone complexes [W(CO)5‐E(PH3)2] (W‐3 E) show that the effect of bulky ligands may obscure the intrinsic WE bond strength. Opposite bonding trends: Quantum chemical calculations suggest that the bond strength of WE (E=C–Pb) in the experimentally yet unknown tetrylone complexes W‐1 E increases for the heavier Group 14 elements from carbon to lead, whereas the WE bond in the tetry