Chemical Bonding in [Fe(η⁴-P₄)₂]^2– and Related Complexes

Quantum chemical calculations of the six valence isoelectronic complexes [FeL₂]^2-, [CoL₂]^-, and NiL₂ with L = η⁴-P₄, η⁴-C₄H₄ using density functional theory have been carried out. The molecular structures were investigated with a variety of methods. The analysis of the electronic structure in [Fe(η⁴-P₄)₂]^2- shows that the bonding situation is very similar to valence isoelectronic Ni(η⁴-C₄H₄)₂. The orbital interactions in the 18 electron complexes [TML₂]^q (TM^q = Fe^2-, Co^-, Ni) come mainly from TM(d_π)→L₂ backdonation, enhanced by smaller contributions from TM(d_δ)→L₂ backdonation and TM(s)←L₂ donation. Calculations of the six TML₂ species indicate that all of them are viable candidates for synthetic work. The bonding situation is very similar and can straightforwardly be explained with the Dewar-Chatt-Duncanson bonding model in terms of dative bonding between d¹⁰ metal atoms and the ligands in the electronic singlet state. EDA-NOCV calculations using the ligands and the metal atoms with different charges and electronic states indicate that the metal-ligand bonds in the charged complexes [FeL₂]^2- and [CoL₂]^- are best described with fragments in the electronic triplet state between the metal atoms with d⁸ configuration and triplet ligands. The singlet fragments give the degenerate TM(d_π)→L₂ π backdonation as the strongest component, whereas the triplet fragments have the related electron-sharing TM^q (d_π)-(L₂)^2- π bonding as the major component, differing only by the assignment of the bonded two electrons to one or both fragments. The calculations of the charge distribution using the Hirshfeld and Voronoi partitioning methods suggest that the metal atoms are nearly neutral or carry small negative charges in all complexes. The NBO method gives erratic charges, because of the neglect of the 4p AOs of the transition metals as genuine valence orbitals.