Group 10 Bis(iminosemiquinone) Complexes: Measurement of Singlet–Triplet Gaps and Analysis of the Effects of Metal and Geometry on Electronic Structure

Bis(iminosemiquinone) complexes of divalent group 10 metals have been described as having open-shell singlet ground states characteristic of very strong coupling between the two ligand radicals. By using the nonlinear temperature dependence of the chemical shifts of the 1H NMR spectra, the singlet-triplet gaps in seven of these compounds have been measured, with the nickel compounds having gaps of about 2400 cm-1 and the palladium compounds about 1800 cm-1. Bis(iminosemiquinone)platinum complexes have singlet-triplet gaps too large to measure by this technique (over 2800 cm-1, estimated to be about 3000 cm-1), though bis(3,5-di- tert-butylbenzosemiquinonato)platinum(II) has a measurable singlet-triplet gap of 1850 cm-1. In combination with near-IR absorption data of the neutral, cationic, and anionic bis(iminosemiquinone) complexes, a simplified two-electron, two-orbital bonding model describing these compounds can be fully parametrized based on experimental data. The identity of the central metal principally affects the difference in energy between metal-ligand π nonbonding and metal-ligand π antibonding orbitals, with the strength of the bonding interactions increasing in the order Pd < Ni < Pt. Twisting the ligands out of planarity (by using a 2,2'-biphenylenediyl linker) has a marked effect on the optical spectra of the compounds but not on their singlet-triplet gaps; this indicates that the effect is not due to changes in bonding interactions but rather due to a decrease in the magnitude of the quantum mechanical exchange interactions in the twisted compared to the flat compounds.