Cooperativity in the Iron(II) Spin-Crossover Compound [Fe(ptz)(6)](PF(6))(2) under the Influence of External Pressure (ptz = 1-n-Propyltetrazole).

The iron(II) spin-crossover compound [Fe(ptz)(6)](PF(6))(2) (ptz = 1-propyltetrazole) crystallizes in the triclinic space group P&onemacr;, with a = 10.6439(4) Å, b = 10.8685(4) Å, c = 11.7014(4) Å, alpha = 75.644(1) degrees, beta = 71.671(1) degrees, gamma = 60.815(1) degrees, and Z = 1. In [Fe(ptz)(6)](PF(6))(2), the thermal spin transition is extremely steep because of cooperative effects of elastic origin. The transition temperature at ambient pressure is 74(1) K. An external pressure of 1 kbar shifts the transition temperature to 102(1) K, corresponding to a stabilization of the low-spin state, which is smaller in volume. The volume difference between the high-spin and the low-spin state, DeltaV degrees (HL), is 24(2) Å(3)/molecule. The interaction constant Gamma, as a measure of cooperativity, is within experimental error independent of external pressure and has a value of 101(5) cm(-)(1). In contrast to the case of the related compound [Fe(ptz)(6)](BF(4))(2) (Decurtins et al. Inorg. Chem.1985, 24, 2174), there is no hysteresis due to a first-order crystallographic phase transition, nor is there a hysteresis induced by external pressure as in the mixed crystal [Zn(1)(-)(x)()Fe(x)()(ptz)(6)](BF(4))(2), x = 0.1 (Jeftic et al. J. Phys.Chem. Solids 1996, 57, 1743). However, in [Fe(ptz)(6)](PF(6))(2), the interaction constant Gamma is found to be very close to the critical value above which a hysteresis solely due to the cooperative effects is expected. In addition, high-spin --> low-spin relaxation measurements were performed under external pressures of up to 1 kbar in the temperature interval between 50 and 60 K. An external pressure of 1 kbar accelerates the high-spin --> low-spin relaxation by 1 order of magnitude.