The influence of Jahn-Teller coupling on the high-spin/low-spin equilibria of octahedral MIIIL6 polyhedra (MIII: Mn - Cu), with NiF63 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 Eg ⊗ εg 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 T2g ⊗ ε2g coupling - here contributing to the stabilisation of the low spin 2T2g and the high spin 5T2g 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 NiF63-, adopting additionally to spectroscopic, magnetic and structural results, reliable data from DFT, that the minimum positions of the alternative, ground state potential curves differ by only Δ2,4 {all equal to} 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 Δ2,4. 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 Nd0.8Sr1.2NiIIIO3.9 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 Eg ⊗ εg JT coupling. Thus, CoF63- is high-spin, while oxygen-ligator atoms induce challenging high-spin/low-spin equilibria, which are discussed and analysed. The high-spin (5Eg)/low-spin (3T1g) separation energy for MnIII bears a different sign in comparison to NiIII, 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 Oh between the lowest energy excited, states, launching considerable lowering in energy of the, split state - the new ground state in D4h. Indeed, while the CuF63- octahedron is high-spin, the less ionic oxygen ligand usually induces a (nearly) square-planar CuIIIO4 coordination. The CuIII-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.