Electronic Structure and Spectroscopy of the Cycloheptatrienyl Molybdenum Halide Complexes

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)] (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-y2 orbitals resulting in a metal-centred HOMO with substantial dz2 character in the 18-electron, closed shell systems. The experimental electronic UV-visible 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 (metalligand 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