Color Properties and Structural Phase Transition in Penta- And Hexacoordinate Isothiocyanato Ni(II) Compounds

We investigated the optical properties of (NBu4)3[Ni(NCS)5], a pentacoordinate Ni compound, and compared the results with the more traditional hexacoordinate analogue (NEt4)4[Ni(NCS)6]. On the basis of our complementary electronic structure calculations, the color properties of this high spin complex can be understood in terms of excitations between strongly hybridized orbitals with significant Ni d and ligand character. Variable temperature vibrational studies show mode softening with decreasing temperature and splitting near 200 K, trends that we attribute to improved low temperature intermolecular interactions and a weak structural phase transition, respectively.