Interpretation of the temperature dependence of the crystal structure of [CuL2][BF4](2) (L=2,6-dipyrazol-1-ylpyridine)

The temperature dependence of the bond lengths in the high temperature phase of [Cu(L)(2)][BF4](2) (L = 2,6-dipyrazol-1-ylpyridine) may be interpreted satisfactorily using a model of dynamic, Jahn-Teller (JT) vibronic coupling. The geometries of the three complexes in the asymmetric unit of the low temperature phase may be reproduced using parameters similar to those of the high temperature phase, but with significantly larger orthorhombic components of the strain interactions. The axial components of the strain for all the complexes in both phases are negative in sign and large in magnitude, and this is thought to reflect both the stronger sigma-bonding power of pyridine compared with pyrazole and physical constraints of the amine ligand. A previous interpretation of the thermal behaviour of the high temperature phase in terms of a conformational equilibrium is re-evaluated.