The reaction of solvent substituted MoO2X2(S)(2) (X = Cl, S = THF; X = Br, S = DMF) complexes with one equivalent of bidentate nitrogen donor ligands at room temperature leads within a few minutes to the quantitative formation of complexes of the type [MoO2X2L2] (L = 4,4'-bis-methoxycarbonyl-2,2'-bipyridine, 5,5'-bis-methoxycarbonyl-2,2'-bipyridine, 4,4'-bis-ethoxycarbonyl-2,2'-bipyridine, 5,5'-bis-ethoxycarbonyl-2,2'-bipyridine). Treatment of the complexes [MoO2Cl2L2] with Grignard reagents at low temperatures yields dimethylated complexes of the formula [MoO2(CH3)(2)L-2]. [MoO2Br2(4,4'-bis-ethoxycarbonyl-2,2'-bipyridine)], [MoO2Br2(5,5'-bis-methoxycarbonyl-2,2'-bipyridine)] and[MoO2Br2(5,5'-bis-ethoxycarbonyl-2,2'-bipyridine)] have been exemplary examined by single crystal X-ray analysis. The complexes were applied as homogenous catalysts for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) as oxidising agent under solvent-free conditions. The complexes containing L = Cl have been additionally investigated with room temperature ionic liquids (RTILs) as solvents. The catalytic activity of the [MoO2X2L2] complexes in olefin epoxidation with tert-butyl hydroperoxide is on average very good. The main advantage of the synthesised complexes in comparison to previously reported complexes is their high solubility. This good solubility is apparently the reason that the catalytic potential of the compounds can unfold. The turnover frequencies (TOFs) in RTILs are even higher, showing the performance of the catalysts under optimised conditions. (C) 2010 Published by Elsevier B.V.