D-orbital energy levels in planar [MIIF4]2−, [MII(NH3)4]2+ and [MII(CN)4]2− complexes: the nature of M–L π bonding and the implications for ligand field theory. Issue 28 (3rd July 2020) - Deeth, Robert J.
Abstract : The 'coordination voids' above and below the molecular plane exert significant σ and π ligand field effects. Abstract : Qualitative MO theory predicts degenerate dπ orbitals for planar coordination complexes with formally σ-only ligands and the splitting energy, Δ E π = E (d xy ) − E (d xz, d yz ), should be zero. For π-donor ligands, Δ E π should be positive (d xy > d xz, yz ) while for π-acceptors, Δ E π should be negative (d xy < d xz, yz ). However, experimental d–d spectra, ab initio ligand field theory (AI LFT) and crystal field theory for σ-only [M(NH3 )4 ] 2+ complexes give pronounced dπ splittings with Δ E π around +2500 cm −1 for first-row, divalent metal ions. AI LFT further suggests Δ E π values around +4500 cm −1 for [MF4 ] 2− and +1000 cm −1 for [M(CN)4 ] 2− species. The origins of these dπ orbital splittings can be traced to the effects of the ligand field potential surrounding the metal centre which includes not only the intrinsic metal–ligand π bonding but also substantial contributions from the 'void' regions above and below the molecular plane. The π component of the 'void cell' potentials increases Δ E π which, if not explicitly taken into account, artificially enhances the apparent π-donor strength of the ligands. With the inclusion of void cell π interactions, even though the AI LFT d orbital sequence always places d xz, yz below d xy, the ligand field analysis provides a chemically-reasonable description of the M–L π interactions with cyanide being a weak π acceptor, ammonia being π-neutral and fluoride being a strong π donor. In the case of [Ni(CN)4 ] 2−, ligand field calculations further show that, contrary to the recent claims of Oppenheim et al. ( Inorg. Chem., 2019, 58, 15202) the sequence of the many-electron excited states is not a definitive guide to the underlying order of one-electron d orbital energies and that the observed sequence of n A2g > n Eg > n B1g, n = 1 or 3, does not guarantee a d-orbital sequence of d xy < d xz, yz < d z 2 .