Metal-metal bonding in mixed valence Ni25+ complexes and spectroscopic evidence for a Ni26+ species

Dinickel(II) complexes of the ligands N,N'-di-p-anisylformamidinate (DAniF) and N, N',N ''-triphenylguanidinate (TPG) have been synthesized and crystallographically characterized, along with their one-electron-oxidized analogues. In both systems, the Ni - Ni distances become shorter by similar to 0.1 angstrom upon oxidation, in accord with the proposal that the resulting Ni-2(5+) complexes are appropriately described as having one electron removed from a metal-based sigma* orbital and an overall Ni - Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni-2(HNCHNH)(4) and [Ni-2(HNCHNH)(4)](+) appear to predict that the lowest energy state of the latter species would have one unpaired electron in an essentially ligand-based orbital. A single-point calculation of Ni-2(DAniF)(4) employing the geometry of its crystal structure with the full ligand included reveals a reversal of the previously predicted order of the HOMO and HOMO-1, and suggests that the unpaired electron in [Ni-2(DAniF)(4)](+) is in a metal-based orbital of sigma* symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni-2(DAniF)(4) shows an unexpectedly rich cyclic voltammogram with four stepwise reversible oxidation waves. Coulometric experiments show that the doubly oxidized species has a significant lifetime at - 25 degrees C, and by spectroelectrochemistry, its UV - vis spectrum was recorded. We propose that this species contains a Ni-2(6+) core with a single Ni - Ni sigma bond.