Co(II) molecular complexes as a reference for the spin crossover in Fe(II) analogues

The crystal structures of a series of cobalt(II) molecular complexes, [Co(PM-L)(2)(NCS)(2)] [PM=N-2-pyridylmethylene, L=4-(aminobiphenyl) or 4-(phenylethynyl)aniline], are investigated and compared to the analogous iron(II) complexes, [Fe(PM-L)(2)(NCS)(2)], already known in the literature. At room temperature, the Co(II) complexes prove to be isostructural with the iron(II) complexes. An interesting point is that the iron complexes, unlike the cobalt complexes, undergo a spin crossover at low temperature. Hence, a comparison of the temperature dependence of the structural properties of the Co(II) and the Fe(II) complexes underlines some structural features of the spin crossover. Comparative deformation of the lattices and thermal expansion tensors are first discussed. Then, new parameters to estimate the distortion and the contraction at the spin crossover of the FeN6 coordination sphere are presented, thereby allowing the estimation of the reduction of the volume of the octahedron to around 3 Angstrom(3) (25%). As well, comparative discussions on the intermolecular contact modifications with temperature are proposed. In the above considerations the cobalt series is therefore used as a reference to distinguish between the effects of the spin crossover and the purely thermal effects.