Thermally induced spin-crossover in the Fe(3-ethynylpyridine)2[M(CN)4] series with M = Ni, Pd, and Pt. The role of the electron density found at the CN 5σ orbital

The charge density accumulated at the CN 5σ orbital, via π-back bonding interaction, determines the crystal field splitting for the Fe( ii ) atom and the temperature where the spin transition is observed.