NUCLEOPHILIC REACTIONS OF CHIRAL HETEROBIMETALLIC COBALT-MOLYBDENUM PROPARGYLIUM COMPLEXES, INVOLVING THE FIRST EXAMPLE OF PHOSPHINE MIGRATION - X-RAY CRYSTAL-STRUCTURES OF [(MU-ETA(2)-ETA(3)-HC-CCH(CH3))COMOCP(CO)(5)][BF4] AND )-ETA(3)-HC-CCH(CH)(3))COMOCP(CO)(4)(PPH(3))][BF4]

Chiral complexes of the type )C(1)=(CCH)-C-2-H-3(R(3)))CoMoCp(CO)(5)][ BF4][R(1) = R(3) = H (2); R(1) = H, R(3) = CH3 4a,b; R(1) = CH3, R(3) = H (6)] have been synthesized and characterized. These carbenium ions were obtained via the protonation of the corresponding alcohol or eth er complexes 1)C(1)=(CCH)-C-2-H-3(R(3))(OH))CoMoCp(CO)(5)][R(1) = R(3) = H (1); R(1) = R(3) = CH3 (3a,b)] or C(1)=(CCH)-C-2-H-3(R(3))(OCH3)) CoMoCp(CO)(5)][R(1) = CH3, R(3) = H (5)]. The diastereomeric alcohols 3a,b give the same mixture of cations (mu-eta(2)-eta(3)-HC=CCH(CH3))-C oMoCp(CO)(5)][BF4] (4a,b) with a moderate diastereoselectivity (85/15) . These cations react with sodium methanethiolate to give a mixture of thioethers [(mu-eta(2)-eta(2)-HC=CCH(CH3)SCH3)-CoMoCp(CO)(5)] (9a,b), in the same diastereoselective ratio relative to the starting materia l. The reaction with triphenylphosphine leads to the expected phosphon ium derivative ta(2)-eta(3)-H3CC=CCH2(PPh(3)))CoMoCp(CO)(5)][BF4] (10) in the case of the primary cation. With the secondary cation (4a) the initially formed phosphonium ion (3)-HC=CCH(CH3)(PPh(3)))Co(CO)(3)MoC p(CO)(2)][BF4] (11a,b) is readily transformed to a cobalt-bonded triph enylphosphine, involving a phosphine migration from the carbenium cent er to the cobalt center, [(mu-eta(2)-eta 3-HC=CCH(CH3))Co(CO)(2)(PPh(3 ))MoCp(CO)(2)][BF4] (12a,b). Structural and mechanistic models are pro posed to explain the diastereoselectivity observed in the formation of the phosphinated carbenium ions (12a,b). The X-ray crystal structure of (3R, cluster R*)-(4a) has been determined. It is the first describ ed structure for a secondary carbenium center possessing a chiral clus ter in the ct-position. The X-ray structure of (3R, cluster R*)-(12a) was also determined and provides evidence that the phosphine ligand i s bonded to the cobalt metallic center.