Specific examples of suitable M??? groups include monovalent zinc or aluminum radicals and Lewis base containing complexes thereof, most preferably ???Zn(PG) and ???Al(PG)2, wherein PG is a protecting group, preferably a group selected from hydrogen, halo, hydrocarbyl, tri(hydrocarbyl)silyl, halo-substituted hydrocarbyl, and halo-substituted tri(hydrocarbyl)silyl.