wherein R11 and R12 are each an alkyl group having 1 to 8, preferably 1 to 4 carbon atoms, X' is a hydrogen atom or a halogen atom, p satisfies the relation 0<p???3, desirably p=2 or 3, most desirably p=3, and q satisfies the relation 0???q<3, desirably q=0 or 1; the magnesium compound having an alkyl group represented by the general formula (VIII)