title: Metal-metal bonding in mixed valence Ni2(5+) complexes and spectroscopic evidence for a Ni2(6+) species.
abstract: 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 approximately 0.1 A upon oxidation, in accord with the proposal that the resulting Ni2(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 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 sigma 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 degrees C, and by spectroelectrochemistry, its UV-vis spectrum was recorded. We propose that this species contains a Ni2(6+) core with a single Ni-Ni sigma bond.
authors.affiliation: Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012, Texas A and M University, College Station, Texas 77842-3012, USA.
authors.forename: John F
authors.initials: JF
authors.lastname: Berry
authors.forename: Eberhard
authors.initials: E
authors.lastname: Bothe
authors.forename: F Albert
authors.initials: FA
authors.lastname: Cotton
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Ibragimov
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Murillo
authors.forename: Dino
authors.initials: D
authors.lastname: Villagran
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wang