Differential reactivity of the dihydrides Mn2(.mu.-H)2(CO)6(.mu.-L-L) (L-L = (EtO)2POP(OEt)2, Ph2PCH2PPh2) toward Fe2(CO)9 or M(CO)6 (M = Cr, Mo, W). X-ray crystal structure of Mn2Fe(.mu.-H)(.mu.-PhPCH2PPh2)(CO)10