Explanation of the site-specific spin crossover in Fe(mtz)(6)(BF4)(2)

The spin crossover behavior of the two [Fe(mtz)(6)](2+) complexes occupying different lattice sites in Fe(mtz)(6)(BF4)(2) is addressed by combining quantum chemical calculations with a careful analysis of the crystal structure. It is first established from the calculations that the energy difference between high spin and low spin states depends on the orientation of the tetrazole ligands; small rotation angles favor the low spin state, while for angles larger than similar to 20 degrees the high spin state is more stable. The crystal structure shows that the two complexes have different average rotation angles of the ligands. It is larger for the site that remains HS down to low temperatures and smaller for the site that shows spin crossover to LS. The origin of the different rotation angles is found to be determined by a subtle interplay amongst steric repulsion between the ligands, H...F interactions between the complex and the counterions, and intersite interactions involving N...H contacts and pi-pi interactions between the N=N double bonds of the tetrazole rings.