Electronic Structure of Linearly Coordinated EQ Complexes of the Type [(N₃N)W(EQ)] [N₃N = N(CH₂CH₂NSiMe₃)₃; E = P, As, Sb, Bi; Q = O, S, Se, Te]: A DFT Study

Abstract

Density functional theory (DFT) calculations were carried out on the terminal EQ complexes [(N3N)W(EQ)] {N3N = N(CH2CH2NSiMe3)3; E = P, As, Sb, Bi; Q = O, S, Se Te} to clarify the bonding situation within the linear Nax–W–E–Q core. This unusual structural motif gives rise to a bonding arrangement in which the π‐electron density is delocalised over the three atoms of the W–E–Q unit. Fragment calculations and natural bond order (NBO) data indicated that the σ‐bonding component of the Nax–W–E–Q unit comprises two occupied σ orbitals, while the π component of bonding comprises two sets of degenerate π orbitals. In general, the π orbitals of the Nax–W–E–Q core are higher in energy compared to the σ orbitals. The phosphorus monoxide (EQ = PO) complexes provide an exception to this rule, with the 1π orbitals of the W–P–O core lower in energy than the σ orbitals. Generally, as the atomic number of either the pnicogen (E) or chalcogen (Q) atom increases the extent of σ‐orbital delocalisation decreases, whereas the π‐orbital delocalisation increases. Fractional bond orders and Wiberg bond indices were used to establish whether localisation of the π‐electron density gives rise to a W–E or an E–Q double or triple bond. Both methods indicate a W–E as well as an E–Q double bond. The ionic nature of the complexes were analysed by inspection of the Hirschfeld charge distribution which shows only a moderate ionic character. Exceptions are the pnicogen monoxide complexes, which are more ionic. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)