Electron paramagnetic resonance spectra of rhenium tetrachloride oxide and some of its adducts

Abstract

X-Band e.p.r. spectra of Re(O)Cl₄, [Re(O)Cl₄(NCMe)], [Re(O)Cl₄(OPCl₃)], and [Ph₄As][Re(O)Cl₅] have been recorded at 77 K and are analysed in detail. The metal-ion spin–orbit coupling constant and the parameter P= 2·002g_Reβ_eβ_N〈ψ|r^–3|ψ〉 have been estimated to be ca. 2 400 and 0·033 cm^–1 respectively. Spin-Hamiltonian parameters are listed for each substance and are equated to the atomic-orbital coefficients in some of the molecular orbitals involved in bonding in these molecules. The unpaired electron lies in the metal-ion 5d_xy orbital and is strongly delocalised (33%) on to the chloride ligands. g∥ Is greater than g⊥ and this is shown to be essentially due to charge-transfer mixing through spin–orbit coupling at the chlorine atoms. Changes in the spin-Hamiltonian parameters when a sixth ligand is added to Re(O)Cl₄ are accounted for. Hückel LCAO molecular-orbital calculations on Re(O)Cl₄ appear to give a reasonable description of its ground state.