Spin states at a tipping point:: What determines the dz2l ground state of nickel(III) Tetra(tbutyl)porphyrin dicyanide?

Density functional theory (DFT) calculations, regardless of the exchange-correlation functional, have long failed to reproduce the observed d(z2)(1) ground state of the [Ni-III(T'BuP)(CN)(2)](-) anion (where (TBuP)-Bu-t is the strongly ruffled tetra((t)butyl)porphyrin ligand), predicting instead a d(x2-y21) ground state. Normally, such failures are associated with DFT calculations on spin states of different multiplicity, which is not the case here. The calculations reported here strongly suggest that the problem does not lie with DFT. Instead environmental factors need to be taken into account, such as counterions and solvents. Counterions such as K+ placed against the cyanide nitrogens and polar solvents both result in a d(z)(21) ground state, thus finally reconciling theory and experiment.