Inorganic Chemistry, Vol.58, No.22, 15202-15206, 2019
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 < z(2) < z < x(2)-y(2) < x, y and assign the d-d bands in the absorption spectrum to (1)A(1)g -> B-3(1g) < E-3(g) < (3)A(2g) < B-1(1g) < E-1(g) < (1)A(2g). While differing from all previous interpretations, our assignments accord with an MO model in which strong pi-backbonding in the plane of the molecule stabilizes d(xy) more than out-of-plane bonding stabilizes d(xz) and d(yz).