Ligand field theory, Pauli shields and ultra-covalency in organometallic chemistry

Abstract

This paper explores the ligand field picture applied to organometallic compounds. Given the dearth of experimental data, the high-level ab initio ligand field theory (aiLFT) method is deployed as a surrogate for experiment and the necessary d orbital sequences and relative energies are obtained computationally. These are fitted to local cellular ligand field (CLF) σ, π and δ bonding parameters. Results are reported for planar [Cu(CR₃)₄]⁻, (R = F, H), octahedral M(CO)₆ⁿ (M = Fe, Mn, Cr, V, Ti; n = +2, +1, 0, −1, −2), and the sandwich compounds M(Cp)₂ (Cp = cyclopentadienyl, M = Fe, Ni, V), [Ni(Cp)₂]²⁺ and Cr(C₆H₆)₂. With respect to the aiLFT framework, these organometallic systems behave just like coordination complexes and most maintain the integrity of their formal dⁿ configurations. Both [Cu(CR₃)₄]⁻ compounds are formulated as low-spin d⁸ Cu^III species and have normal ligand fields consistent with their planar geometries. The metal carbonyls reveal a new way of counting valence electrons which only requires the CLF d orbital energy level diagram to rationalise the 18-electron rule as well as its many exceptions. The bonding in sandwich compounds shows a remarkable variation. In ferrocene, Cp⁻ behaves as a strong field ligand, comparable to [CN]⁻ in [Fe(CN)₆]⁴⁻. Fe(Cp)₂ is low spin as is Cr(C₆H₆)₂. Cp⁻ in Fe(Cp)₂ is a weak σ donor, strong π donor and weak δ acceptor while benzene in Cr(C₆H₆)₂ is also a weak σ and strong π donor but is a much better δ acceptor. In contrast, Cp⁻ is weak field in high spin, 20-electron Ni(Cp)₂ but ‘ultra-covalent’ in [Ni(Cp)₂]²⁺. The formal IV oxidation state is too high for the ligand set and the integrity of the d⁶ configuration is lost. Similarly, [V(CO)₆]⁻ and [Ti(CO)₆]²⁻ are ultra-covalent except now the formal metal oxidation states are too negative. Both mechanisms relate to the breaching of the metal's 3s²3p⁶ ‘Pauli shield’ and these ultra-covalent systems lie outside the ab initio ligand field regime. However, within the ligand field regime, the bonding in ‘coordination complexes’ and ‘organometallic compounds’ has the same conceptual footing and the nature of the local σ, π and δ interactions can be extracted from analysing the ligand field d orbitals.