DFT calculations at the B3LYP/Def2-SVP level have been conducted on the half-sandwich cycloheptatrienyl molybdenum complexes [Mo(CO)(3)(eta-C7H7)(+), (+) and [MoBrL2(eta-C7H7)](n+) (L-2= 2 CO, n = 0, 2; L-2= bpy, n = 0, 3; L-2 = bpy, n = 1, (+); bpy = 2,2'-bipyridyl). In all cases, strong delta-bonding interactions operate between the e(2) level of the C7F7 ring and metal d(xy), and d(x2-y2) orbitals resulting in a metal-centred HOMO with substantial d(z2), character in the 18-electron, closed shell systems. The experimental electronic UV-Vis spectra of (+), 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. (C) 2014 Elsevier Ltd. All rights reserved.