Electronic Structure of Tetracyanonickelate(II)

Tetracyanonickelate(II) has been a poster child of ligand field theory for several decades. We have revisited the literature assignments of the absorption spectrum of [Ni(CN)₄]^2- and the calculated ordering of orbitals with metal d character. Using low-temperature single-crystal absorption spectroscopy and accurate ab initio and density functional quantum mechanical methods (NEVPT2-CASSCF, EOM-CCSD, TD-DFT), we find an ordering of the frontier d- and p-orbitals of xy < xz, yz < z² < z < x²-y² < x, y and assign the d-d bands in the absorption spectrum to ¹A_1g → ³B_1g < ³E_g < ³A_2g < ¹B_1g < ¹E_g < ¹A_2g. While differing from all previous interpretations, our assignments accord with an MO model in which strong π-backbonding in the plane of the molecule stabilizes d more than out-of-plane bonding stabilizes d and d.