VO(OR)aXb or V(OR)cXd In the above formulas, R is a hydrocarbon group, such as an alkyl group, a cycloalkyl group or an aryl group; X is a halogen atom; and a, b, c and d are numbers satisfying the conditions of 0???a???3, 0???b???3, 2???a+b???3, 0???c???4, 0???d???4 and 3???c+d???4.