Electronic structure and spectroscopy of the cycloheptatrienyl molybdenum halide complexes [MoBrL2(η-C7H7)]n+ (L2 = 2CO, n = 0; L2 = 2,2′-bipyridyl, n = 0 or 1)

DFT calculations at the B3LYP/Def2-SVP level have been conducted on the half-sandwich cycloheptatrienyl molybdenum complexes [Mo(CO)3(η-C7H7)]+, [1]+ and [MoBrL2(η-C7H7)]n+ (L2 = 2 CO, n = 0, 2; L2 = bpy, n = 0, 3; L2 = bpy, n = 1, [3]+; bpy = 2,2′-bipyridyl). In all cases, strong δ-bonding interactions operate between the e2 level of the C7H7 ring and metal dxy   and dx2-y2dx2-y2 orbitals resulting in a metal-centred HOMO with substantial dz2dz2 character in the 18-electron, closed shell systems. The experimental electronic UV–Vis spectra of [1]+, 2 and 3 are accurately reproduced by TD-DFT methods. For complexes 2 and 3, assignments made with the assistance of calculated spectra indicate that absorptions in the region 390–770 nm originate from a series of MLCT (metal–ligand charge transfer) or ILCT (inter-ligand charge transfer) transitions in which carbonyl, C7H7 and 2,2′-bipyridyl ligands act as acceptor systems from the metal or mixed metal and bromide donor groups. The metal-centred, one-electron oxidation of 3 to 3[PF6] results in almost complete quenching of the visible region MLCT/ILCT absorptions of 3 and replacement with weak transitions probably arising from bromide to metal LMCT (ligand to metal charge transfer) processes.

Electronic structure calculations (DFT, B3LYP/Def2-SVP) on the half-sandwich cycloheptatrienyl molybdenum complexes [MoBrL2(η-C7H7)] (L2 = 2 CO or 2,2′-bipyridyl) reveal that in each case the HOMO features significant metal dz2dz2 character but the composition of the LUMO is L2 dependent. TD-DFT methods have been employed to analyse the experimental UV–Vis electronic absorption spectra of these complexes.Figure optionsDownload as PowerPoint slide