BIMETALLIC COMPLEXES WITH CHIRAL MOLYBDENUM CENTERS AND BIS(ETA-5-CYCLOPENTADIENYL) BRIDGES - INTERCHANGE BETWEEN LEGS IN 3-LEGGED PIANO STOOL COMPLEXES

Reactions of [{Mo(CO)(3)Cl}(2)(mu-CpCp)] (CpCp = (eta-(C5H4)-C-5)(2)Si Me(2) (1a) or (eta(5)-C5H3)(2)(SiMe(2))(2) (1b)) with AgBF4 and with 2 -butyne in THF (THF = tetrahydrofuran) give [{Mo(CO)(eta(2)-MeCCMe)(2) }(2)(mu-CpCp][BF4](2) (2a,b). Addition of PPh(3) or PPh(4)Cl to 2a or 2b results in the substitution of one 2-butyne ligand at each Mo cente r by PPh(3) or Cl-, giving ionic -(Co)(eta(2)-MeCCMe)(PPh(3))}(2)(mu-C pCp)][BF4](2) (3a,b) or neutral [{Mo(CO)(eta(2)-MeCCMe)Cl}(2)(mu-CpCp) ] (5), respectively. Addition of dmpe (dmpe = dimethylphosphineethane) to 2a gives the free carbonyl complex [{Mo(eta(2)-MeCCMe)(dmpe)}(2)(m u-CpCp)][BF4](2) (4a). Complexes 3 and 5 are obtained as a ca. 1:1 mix ture of the RS isomer and the RR,SS racemate. The RR,SS racemate of 3a can be obtained >90% pure by slow crystallization of the 3a diastereo meric product mixture. In the absence of free ligands, conversion of R R,SS-3a into RS-3a is a first-order reaction with k = (8 +/- 1) x 10(- 5) s(-1) and Delta G(double dagger) = 94.6 +/- 0.2 kJ mol-l at 293 K; and the basic mechanism is likely to be intramolecular.