Electronic structure of four-coordinate C3v nickel(II) scorpionate complexes:: Investigation by high-frequency and -field electron paramagnetic resonance and electronic absorption spectroscopies

A series of complexes of formula Tp*NiX, where Tp*(-)) hydrotris( 3,5-dimethylpyrazole) borate and X) Cl, Br, I, has been characterized by electronic absorption spectroscopy in the visible and near-infrared ( NIR) region and by high-frequency and -field electron paramagnetic resonance ( HFEPR) spectroscopy. The crystal structure of Tp*NiCl has been previously reported; that for Tp*NiBr is given here: space group) Pmc2(1), a) 13.209( 2) angstrom, b) 8.082( 2) angstrom, c) 17.639( 4) angstrom, alpha = beta = gamma = 90 degrees, Z = 4. Tp*NiX contains a four- coordinate nickel( II) ion ( 3d(8)) with approximate C-3v point group symmetry about the metal and a resulting S= 1 high-spin ground state. As a consequence of sizable zero-field splitting ( zfs), Tp*NiX complexes are "EPR silent" with use of conventional EPR; however, HFEPR allows observation of multiple transitions. Analysis of the resonance field versus the frequency dependence of these transitions allows extraction of the full set of spin Hamiltonian parameters. The axial zfs parameter for Tp*NiX displays pronounced halogen contributions down the series: D) +3.93( 2), -11.43( 3), -22.81( 1) cm(-1), for X = Cl, Br, I, respectively. The magnitude and change in sign of D observed for Tp*NiX reflects the increasing bromine and iodine spin - orbit contributions facilitated by strong covalent interactions with nickel( II). These spin Hamiltonian parameters are combined with estimates of 3d energy levels based on the visible-NIR spectra to yield ligand-field parameters for these complexes following the angular overlap model ( AOM). This description of electronic structure and bonding in a pseudotetrahedral nickel( II) complex can enhance the understanding of similar sites in metalloproteins, both native nickel enzymes and nickel-substituted zinc enzymes.