Spin crossover in a one-dimensional iron chain

The electronic structures of a triazole-bridged Fe(II) chain of (MX₃)_n type are examined employing the extended Hückel approximation. Several aspects of the spin transition in the one-dimensional system are presented using a regular chain model with all Fe-N bonds of 2.166 Å, a contracted chain model with all Fe-N bonds of 2.031 Å, and a bond-length-alternant chain model with long and short Fe-N bonds of 2.176 and 2.031 Å. The relative stability among the high-spin (HS), intermediate-spin (IS), and low-spin (LS) states is compared taking one-electron energies and qualitative electron-electron interactions into account. In the regular chain model, "six-below-four" d-block bands appear, and this chain structure is appropriate for the HS state above the observed transition temperature (T_c). On the other hand, in the alternant chain model, one pair of the e_g-block bands is significantly destabilized as a result of geometry perturbation. Below T_c, the LS diamagnetic state in a sufficiently long chain is likely to be more stable than the IS state, which has an -(S = 2)-(S = 0)-(S = 2)-(S = 0)- structure.