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 1A1g → 3B1g < 3Eg < 3A2g < 1B1g < 1Eg < 1A2g. 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 dxy more than out-of-plane bonding stabilizes dxz and dyz.