Insights into the nature of ME bonds in [(PMe3)4ME(Mes)]+ (M = Mo, W) and [(PMe3)5WE(Mes)]+: a dispersion-corrected DFT study

Abstract

Structures and bonding energy analysis of terminal cationic metal–ylidyne complexes [(PMe₃)₄ME(Mes)]⁺ (M = Mo, W) and [(PMe₃)₅WE(Mes)]⁺ (E = Si, Ge, Sn, Pb) were investigated by DFT, DFT-D3 and DFT-D3(BJ) methods using BP86, PBE and PW91 functionals. The Nalewajski–Mrozek (N–M) bond orders and Pauling bond orders show that the M–E bonds in the studied cationic complexes are essentially ME triple bonds. Atomic orbital populations reveal that the out-of-plane π-bonding in all complexes is stronger than the in-plane π-bonding. The bonding of the M–E σ-bond is quite strong, as is the total M–E π-bond strength, and increases upon going from molybdenum to tungsten. The contribution of the orbital interactions ΔE_orb is significantly larger (58–63%) than the electrostatic contributions ΔE_elstat in all the complexes studied. The absolute values of the bond dissociation energies decrease in the order Si > Ge > Sn > Pb. The D3-dispersion energies with zero-damping are in the range 13.0–17.9 kcal mol⁻¹ (BP86), 7.1–10.6 kcal mol⁻¹ (PBE) and 7.7–10.6 kcal mol⁻¹ (PW91), which are smaller than the corresponding DFT-D3(BJ) energies of 21.0–24.6 kcal mol⁻¹ (BP86), 9.9–13.6 kcal mol⁻¹ (PBE) and 10.6–13.6 kcal mol⁻¹ (PW91). The percentage dispersion-corrections to the bond dissociation energies increase as E becomes heavier. The effects of relativistic core contractions in heavier nuclei, i.e. tungsten and lead, are also evaluated.