Insights into the chemistry of Kubas’ chromium dihydrogen complexes

The structures of the complexes Cr(CO)3(PR3)2, (3) R = cyclohexyl and (4) isopropyl and their labile dihydrogen adducts Cr(η2-H2)(CO)3(PR3)2 (1) R = cyclohexyl and (2) isopropyl are calculated using density functional theory and compared to known structures of 3, Cr(η2-HC-CyPCy2)(CO)3(PCy3) (CSD code JOHNOJ) with an agostic CHCr interaction, and of 2, Cr(η2-H2)(CO)3(PiPr3)2 (CSD LEYCOH) with a very short HH distance. The TPSS functional with Grimme D3 dispersion, with a def2-TZVP basis set on all atoms and with the PCM solvent model for tetrahydrofuran (THF) reproduces fairly accurately the geometries, available vibrational spectra, and enthalpy of dihydrogen coordination in THF solution from literature experiments while other computational settings do not. This is a rare example of the calibration of theory with experiment for the Gibbs energy change in a solution phase reaction involving dihydrogen gas. The agostic form of the molecule is favoured in the crystal by intermolecular CHHC interactions as illustrated using Hirshfeld surface analysis and by the more compact size of this conformation.