SYNTHESIS, MOLECULAR-STRUCTURES, AND FLUXIONAL BEHAVIOR OF THE NOVEL YLIDE COMPLEX [CP2MO2(CO)4(MU-ETA(1)ETA(2) MEC=C=C(H)(PPH3))] STABILIZED BY A BIMETALLIC FRAME

Treatment of the propargyl alcohol complex [Cp2Mo2(CO)4(mu-eta2:eta2-M eC=CCH2OH)] (1) with HBF4-Et2O in ether solution afforded the carbeniu m ion complex [CP2Mo2(CO)4(mu-eta2:eta3-MeC=CCH2)] [BF4] (2). Compound 2 undergoes a nucleophilic attack by several phosphine ligands (PR'3, R' = Et, Ph) at the alpha-carbonium center (-CH2+), thus forming the phosphonium derivatives [Cp2Mo2(CO)4(mu-eta2:eta2-MeC=CHC2PR'3)] [BF4] (3) and 4a. In particular the phosphonium complex [CP2Mo2(CO)4(mu-eta :eta2-HC=CCH2PEt3)] [BF4] (4b) belonging to the previous family has be en characterized by X-ray crystallography (space group Pbca, a = 8.932 -(2) angstrom, b = 15.820(6) angstrom, c = 37.432(4) angstrom, V = 528 9 angstrom3, Z = 8). The triphenyl phosphonium derivative [CP2Mo2(CO)4 (mu-eta2:eta2-MeC=CCH2PPh3)] [BF4] (3) can be deprotonated in cold CH3 -CN by reaction with an excess of DBU (DBU = 1,8-diazabicyclo[5.4.0]un dec-7-ene) to yield the two stereoisomeric metal-stabilized ylides [CP 2Mo2(CO)4{mu-eta1:eta2-MeC=C=C(H)(PPh3)}] (5a,b) in a 2:1 ratio. The s tructure of the major isomer 5a was identified by X-ray analysis (spac e group P2(1); a = 9.024(2) angstrom, b = 30.851(7) angstrom, c = 11.3 78(1) angstrom, V = 3123(9) angstrom3, Z = 4). The fluxional behavior in solution of the complexes [CP2Mo2(CO)4{mu-eta1:eta2-MeC-C=C(H) (PPh 3)}1 (5a,b) was investigated, and a mechanism of exchange is presented and compared to that of the parent cation complex [CP2Mo2(CO)4(mu-eta 2:eta3-MeC=CCH2)][BF4] (2).