Syntheses and molecular structures of an unusual cyclic tetrameric organometallic complex, [(CH3)2Al[.mu.-N(iso-C3H7)2]2Mg(CH3)]4, containing tricoordinate magnesiums, and a linear dimer, [(CH3)2Al[.mu.-N(C2H5)2]2Mg(CH3)]2, containing tetracoordinate magnesiums