In general formula (1), examples of a substituted or unsubstituted monovalent hydrocarbon group, R, are: alkyl groups such as a methyl group, an ethyl group, and a propyl group; cycloalkyl groups such as a cyclopentyl group and a cyclohexyl group; alkenyl groups such as a vinyl group and an allyl group; aryl groups such as a phenyl group and a tolyl group; and said groups in which hydrogen atoms a