On the structure and stability of electron deficient species derived from [Cr(CO)3(arene)]

Electron impact ionisation mass spectrometry has been used to study [Cr(CO)3(arene)], arene=C6H6, C6H4Me2-1,4, C6H2Me4-1,2,4,5 and C6Me6. Successive loss of CO ligands results in the production of electron-deficient complexes [Cr(CO)3−x(C6H6)]+ (where x=0 to 3). Analysis of these species using approximate density functional theory indicates that the loss of CO ligands stabilises high-spin states, the effect being more marked in the cationic species than in their neutral counterparts. The high-spin states also tend to have more symmetric structures than their low-spin counterparts. The calculated value of the second CO dissociation energy is lower than either the first or third, consistent with gas-phase data obtained by electron impact ionisation mass spectrometry, which consistently show more intense peaks for the [Cr(CO)(C6H6)]+ than for [Cr(CO)2(C6H6)]+. The loss of the second CO ligand results in a transition from a doublet to a sextet ground state, and the resultant substantial structural rearrangements may be responsible for the anomalously low dissociation energy.