Chemical Bonding\nin [Fe(η<sup>4</sup>‑P<sub>4</sub>)<sub>2</sub>]<sup>2–</sup> and Related Complexes

Quantum chemical calculations of the six valence isoelectronic\ncomplexes [FeL<sub>2</sub>]<sup>2–</sup>, [CoL<sub>2</sub>]<sup>−</sup>, and NiL<sub>2</sub> with L = η<sup>4</sup>-P<sub>4</sub>, η<sup>4</sup>-C<sub>4</sub>H<sub>4</sub> using density\nfunctional theory have been carried out. The molecular structures\nwere investigated with a variety of methods. The analysis of the electronic\nstructure in [Fe(η<sup>4</sup>-P<sub>4</sub>)<sub>2</sub>]<sup>2–</sup> shows that the bonding situation is very similar\nto valence isoelectronic Ni(η<sup>4</sup>-C<sub>4</sub>H<sub>4</sub>)<sub>2</sub>. The orbital interactions in the 18 electron\ncomplexes [TML<sub>2</sub>]<sup>q</sup> (TM<sup>q</sup> = Fe<sup>2–</sup>, Co<sup>–</sup>, Ni) come mainly from TM(d<sub>π</sub>)→L<sub>2</sub> backdonation, enhanced by smaller contributions\nfrom TM(d<sub>δ</sub>)→L<sub>2</sub> backdonation and\nTM(s)←L<sub>2</sub> donation. Calculations of the six TML<sub>2</sub> species indicate that all of them are viable candidates for\nsynthetic work. The bonding situation is very similar and can straightforwardly\nbe explained with the Dewar-Chatt-Duncanson bonding model in terms\nof dative bonding between d<sup>10</sup> metal atoms and the ligands\nin the electronic singlet state. EDA-NOCV calculations using the ligands\nand the metal atoms with different charges and electronic states indicate\nthat the metal–ligand bonds in the charged complexes [FeL<sub>2</sub>]<sup>2–</sup> and [CoL<sub>2</sub>]<sup>−</sup> are best described with fragments in the electronic triplet state\nbetween the metal atoms with d<sup>8</sup> configuration and triplet\nligands. The singlet fragments give the degenerate TM(d<sub>π</sub>)→L<sub>2</sub> π backdonation as the strongest component,\nwhereas the triplet fragments have the related electron-sharing TM<sup>q</sup> (d<sub>π</sub>)–(L<sub>2</sub>)<sup>2–</sup> π bonding as the major component, differing only by the assignment\nof the bonded two electrons to one or both fragments. The calculations\nof the charge distribution using the Hirshfeld and Voronoi partitioning\nmethods suggest that the metal atoms are nearly neutral or carry small\nnegative charges in all complexes. The NBO method gives erratic charges,\nbecause of the neglect of the 4p AOs of the transition metals as genuine\nvalence orbitals.