Dioxomolybdenum(VI) complexes of the type MoO2X2L containing ethylenediimine ligands were prepared in good yield by reaction of MoO2X2(THF)(2) (X = Cl, Br) with the bidentate ligands Ph2C=NCH2CH2N=CPh2 (PBED) or PhCH=NCH2CH2N=CHPh (BED). The crystal structure of MoO2Br2(BED) was determined by X-ray diffraction. The dichloro complexes were active as catalysts for the reaction of cyclooctene with tert-butyl hydroperoxide (TBHP) at 55 degreesC yielding epoxycyclooctane as the only product up to 7 h after the start of the reaction. The highest initial activity was obtained with MoO2Cl2(BED) (63 mol mol(Mo)(-1) h(-1)). Other olefins were also tested, and activities decreased in the order cyclooctene > cycloclodecene > (R)-(+)-limonene > cyclohexene > trans-2-octene > 1-octene. The dibromo complexes exhibited much lower activities than the dichloro complexes for the epoxidation of cyclooctene. (C) 2004 Elsevier B.V. All rights reserved.