Generation of the 19-electron (18 + .delta.) adducts CpMo(CO)3(L2-P) and CpMo(CO)2(L2-P,P') (Cp = .eta.5-CH3C5H4, .eta.5-C5Ph4H, .eta.5-C5Ph5; L2 = 2,3-bis(diphenylphosphino)maleic anhydride). Crystal structure of the (.eta.5-C5Ph4H)Mo(CO)2L2 complex