The Binding Properties of Terminal and Bridging Fluoride and of Aqua Ligands – a Semiempirical Vibronic Coupling and DFT Study of Mixed‐Ligand Manganese(III) Complexes

Abstract

AbstractA combined vibronic coupling and angular overlap analysis of MIIIL6 complexes, supplemented by basic DFT calculations, is performed where the 3d cations are CrIII, FeIII or the Jahn‐Teller unstable MnIII cation, and the ligands L fluoride – in terminal (Ft) or in bridging (Fb) function – or OH2. Our approach is mostly semiempirical and based on a novel ligand strain model, which utilises available structural and spectroscopic data and mostly deals with mixed‐ligand MnIII complexes, containing simultaneously Ft, Fb and H2O. Surveying the great number of published structural data, some interesting results concerning the differing binding properties are derived. While the total bond covalency is similar for the three ligands, the total bond strength – and hence also the ionic bond increment – follows the sequence: Ft ≫ OH2 > Fb. Partitioning the bond covalency into a part, which measures the ligand‐to‐metal charge flow (a), and an increment stemming from orbital (predominantly σ‐) overlap (b), one obtains: Ft ≫ Fb > OH2 for (a) and the reverse sequence for (b).DFT model calculations on the [MIII(Ft)6]3− polyhedra in a solvent continuum were the basis for the elucidation of reliable ground state potential energy surfaces also for the further investigated complexes. The sometimes only very small energy difference between the involved three minima, generated via the interaction with the Jahn‐Teller active vibrational ϵg mode, led to various orthorhombic distortion conformers in the range between the tetragonal elongation and compression, depending on the strain magnitude imposed on the [Mn(Ft)6]3− parent polyhedron by a Ft‐by‐Fb (or OH2) substitution. It is further remarkable, that the average bond length of the Jahn‐Teller distorted octahedron varies significantly with the extent of the deformation and wether it is elongated or compressed – providing also computational evidence for the experimentally observed additional coupling to the totally symmetric α1g mode (Eg ⊗(ϵg + α1) vibronic interaction). The Jahn‐Teller stabilisation energy of the D4h‐elongated [MnIII(Ft)6]3− complex (absolute minimum) with respect to the regular octahedron can be defined as the difference between the non‐Jahn‐Teller restoring energy(≈ 0.3 eV) and the vibronic energy (≈ 0.65 eV). It is considerable (≈ −0.35 eV) in comparison to the stabilisation in regard to the compressed D4h‐conformer (≅ −0.075 eV).DFT was not successful in calculating d‐d transition energies within the 5Eg ground state of MnIII complexes containing the aqua ligand. Apparently, hydrogen bonds of varying strength to the higher‐sphere chemical environment and with significant energetic retroaction on the MnIII‐OH2 bonds exist, which cannot be correctly simulated by non‐sophisticated DFT.