from 1 to 3 depending upon the valency of metal atom M, L is an alkoxy group or an aryloxy group, and k is an integer of from 0 to 2 depending upon m and the valency of metal atom M. Some of hydrogen atoms in the above organic group such as an alkyl group may be substituted by halogen atoms, and an oxygen atom may be inserted between a carbon-carbon bond in the above organic group such as an alky