Thermally-induced spin-crossover (SCO) in Fe(4-ethynylpyrdine) 2 [Fe(CN) 5 NO]. Why is the SCO observed in 2D ferrous nitroprussides despite the NO–NC repulsive interaction in the interlayer region?

The NO ligand behaves as an electron buffer. During the unit cell contraction and related NO–NC distance reduction, electron density flows from the NO towards the CN5σ orbital of the equatorial CN ligands. This flow is inverse during the LS → HS transition.