Quantum-Admixture Model of High-Spin ↔ Low-Spin Transition for Ferrous Complex Molecules

A quantum-admixture model for the d6 configuration ferrous complex molecules with the high-spin ↔ low-spin transition has been established by using the unified crystal-field-coupling (UCFC) scheme. A general study has been made on the spin transition of octahedrally coordinated d6 complexes, and a special application has been given to an Fe(II) compound FeII(TRIM)2(PhCO2)(ClO4). The results show the following: (i) The quantum picture of the spin transition of a d6 system, such as Fe(II), is much more complex than a simple transition between the pure 5T2g and 1A1g states as usually understood. In practice, owing to spin−orbit coupling, spin is no longer a good quantum number and there is no longer a pure 5T2g or 1A1g state. Each of them splits into substates and each substate is a linear combination of various multiplets. The high-spin → low-spin transition of an octahedrally coordinated d6 ion is practically the crossover of the two lowest substates of 5T2g at the critical point. (ii) At the spin-transition critical point the magnetic moment μeff ≈ 5.22μB, which is obviously different from the simple average of the μeff values of high-spin and low-spin states but near the saturation value. (iii) The calculation of the effective molecular magnetic moment μeff for an octahedrally coordinated Fe(II) ion shows that the μeff−T curve is in good agreement with Lemercier et al.'s experiment and both the low-spin value μeff = 0.51μB and the high-spin value μeff = 5.4μB are comparable with the experimental values 0.76μB and 5.4μB, respectively. (iv) The T dependence of the crystal field parameter Dq in the spin-transition region is approximately linear.