The Influence of Jahn–Teller Coupling on the High-Spin/Low-Spin Equilibria of Octahedral M^IIIL₆ Polyhedra (M^III : Mn − Cu), with NiF₆ ³⁻ as the Model Example

Abstract

The appearance of d⁴–to–d⁸ cations in their respective high- or low-spin ground states is not solely a matter of the interplay between the ligand field strength Δ and the Racah parameters of interelectronic repulsion B and C, but can be steered by additional Jahn–Teller (JT) coupling – as in the d⁴ and d⁷ cases, where vibronic E_g ⊗ ε_g interactions strongly stabilise a high- and low-spin ground state, respectively. Also in octahedral complexes with d5 and d6 cations JT interactions come into play, though only via the much weaker T_2g ⊗ ε_2g coupling – here contributing to the stabilisation of the low spin ² T _2g and the high spin ⁵ T _2g state, respectively. Ni(III) occurs, with so far only one exception, as a low spin-species – in the fluoride case exclusively due to the large energy increment stemming from the tetragonal ground state JT splitting. It is further shown for NiF₆ ^3-, adopting additionally to spectroscopic, magnetic and structural results, reliable data from DFT, that the minimum positions of the alternative $_a^2 A_{1g} (_a^2 E_g ){\rm and }_a^2 A_{2g} (_a^2 T_{Ig} )$ ground state potential curves differ by only $\Delta _{2,4} \cong 130cm^{ - 1}$. The energy barrier, on the other hand, which steers the transformation of low into high spin species with increasing temperature, amounts to about 400 cm^-1. O- and N-ligator atoms, which induce larger Δ and smaller B and C values, considerably enhance the mentioned critical quantity Δ_2,4. Interestingly enough, the distinct tetragonal polyhedron distortion, which accompanies the low spin ground state, vanishes in oxidic host lattices, as soon as oxygen serves as a bridging ligand in the respective structure. Band broadening, which suppresses JT coupling, and distinctly enhanced Ni_O bond covalency characterise the bonding in such phases – for example in the K₂NiF₄-type compound Nd_0.8Sr_1.2NiIIIO_3.9 and in the perovskite LaNiO3, where even metallic conductivity is observed. CoIII possesses a high- to low-spin energy barrier to overcome interelectronic repulsion, which is similar to that for NiIII – but without the strong support by E_{g g} ⊗ ε_g JT coupling. Thus, CoF₆ ^3- is high-spin, while oxygen-ligator atoms induce challenging high-spin/low-spin equilibria, which are discussed and analysed. The high-spin (⁵ E _g)/low-spin (³T_1g) separation energy for Mn^III bears a different sign in comparison to Ni^III, due to a larger spin-pairing energy and a pronounced JT coupling, which both favour the high spin ground state in this case. Accordingly, more covalent ligands, positioned higher in the spectrochemical series than fluoride and oxygen, are needed for the high- to-low-spin flip. The d⁸ configuration of CuIII, finally, represents a unique case in so far, as here the singlet-triplet separation energy can only be overcome via excessively tetragonally elongated octahedra. The effect behind is formally described as a pseudo-JT coupling in O _h between the lowest energy excited $_a^I E_g {\rm and }_a^I A_{1g}$ states, launching considerable lowering in energy of the $_a^I A_{1g} {\rm and (}_a^I E_g )$ split state – the new ground state in D_4h . Indeed, while the CuF_{6 3-} octahedron is high-spin, the less ionic oxygen ligand usually induces a (nearly) square-planar Cu^IIIO4 coordination. The Cu^III-O binding properties in various host lattices are characterised, and discussed in respect to the oxidic mixed-valence copper superconductors. Basis for the discussion in all cases are available structural, magnetic and spectroscopic (ligand field, EPR, XANES) data besides results from theory.