. "The process according to claim 7, wherein the carbene compound has the formula (V): ##STR162## wherein M is tungsten and molybdenum, R5 is a C1-10 alkoxy group or a C6-10 aryl group, and R6 is a C1-10 alkyl group, a C3-10 cycloalkyl group, a C6-10 aryl group, or a hydrogen atom." . . .