. "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. a is preferably a number satisfying the condition of 1>a???3, and c is preferably a number satisfying the condition of 1>c?" . . .