An unusual discontinuity in the thermal spin transition in [Co(terpy)2][BF4]2.

Single crystal and powder samples of [Co(terpy)(2)][BF(4)](2) are low spin below 100 K but show a typically gradual thermal spin-transition on warming, centred near 270 K. However, the spin-crossover exhibits an unusual and pronounced discontinuity above room temperature, when the material is ca. 87% high-spin. A crystallographic study at nine temperatures between 30-375 K showed that the discontinuity is not caused by a phase transition or by changes in anion disorder. Rather, it may reflect the steric consequence of small changes in the nearest neighbour interactions between the cations as the spin transition progresses.