The novel molybdenum(IV) compound [MoO(NCMe)(5)][B(C6E5)(4)](2) (1a) has been prepared by air oxidation of [Mo(CO)(3)(NCMe)(3)] in the presence of [H(OEt2)(2)][B(C6F5)(4)] at room temperature. The [MoO(NCMe)(5)](2+) cation shows a distorted octahedral configuration with two different acetonitrile-metal bond lengths due to a trans effect of the oxo-ligand. The trans-acetonitrile ligand is easily abstracted under oil pump vacuum to form [MoO(NCMe)(4)][B(C6F5)(4)](2) (1b). The complex [MoO(NCPh)(4)][B(C6F5)(4)](2) (2) is formed by substitution of acetonitrile ligands of la with benzonitrile molecules. The Mo(IV) complexes can be applied in the homopolymerization of isobutene at 30 degrees C leading to high yields and moderate molecular weights. (C) 2011 Published by Elsevier Ltd.