TY - CHAP AU - Hauser, Andreas TI - Ligand-field theoretical considerations PY - 2004 BT - Topics in Current Chemistry, Spincrossover in Transition Metal Compounds, Vol I PB - (eds P. Gütlich, H. A. Goodwin), Springer, Berlin VL - 233 SP - 49 EP - 58 M3 - 10.1007/b40394-9 UR - http://dx.doi.org/10.1007/b40394-9 KW - spin crossover KW - ligand field theory KW - optical properties KW - vibronic structure KW - configurational coordinate N2 - The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon. ID - 573 ER -