title: Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure.
abstract: The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni⋅⋅⋅Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.
authors.affiliation: EaStCHEM School of Chemistry and Centre for Science at Extreme Conditions, The University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, EH9 3JJ, UK.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Byrne
authors.forename: Patricia J
authors.initials: PJ
authors.lastname: Richardson
authors.forename: John
authors.initials: J
authors.lastname: Chang
authors.forename: Anna F
authors.initials: AF
authors.lastname: Kusmartseva
authors.forename: David R
authors.initials: DR
authors.lastname: Allan
authors.forename: Anita C
authors.initials: AC
authors.lastname: Jones
authors.forename: Konstantin V
authors.initials: KV
authors.lastname: Kamenev
authors.forename: Peter A
authors.initials: PA
authors.lastname: Tasker
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons