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 ∼0.1 Å upon oxidation, in accord with the proposal that the resulting Ni ₂⁵⁺ complexes are appropriately described as having one electron removed from a metal-based σ* orbital and an overall Ni-Ni bond order of 1/2. Although conventional DFT calculations on the model compounds Ni₂(HNCHNH)₄ and [Ni₂(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 Ni₂(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 [Ni₂(DAniF)₄] ⁺ is in a metal-based orbital of σ* symmetry. This is verified by the axial EPR spectrum of the compound in solution. The compound Ni₂(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°C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni₂⁶⁺ core with a single Ni-Ni σ bond.