REACTIVITY, VARIABLE-TEMPERATURE SOLUTION H-1-NMR STUDIES AND MO CALCULATIONS OF DIMOLYBDENUM ALLENYLIDENE COMPLEXES OF THE TYPE A(5)-C5H5)(2)MO-2(CO)(4)(MU,ETA(2)(4E)-C=C=CR1R2)]

The reactivity, towards nucleophiles and electrophiles, of dimolybdenu m allenylidene complexes of the type [Cp(2)Mo(2)(CO)(4)(mu,eta(2)(4e)- C=C=CR1R2)] (Cp = eta(5)-C5H5) has been investigated. The nucleophilic attacks occur at the C-gamma carbon atom, while electrophiles affect the C-alpha atom. Variable temperature solution H-1 NMR studies show a dynamic behavior of these complexes consisting of an equilibrium betw een two enantiomers with a symmetrical [Cp(2)Mo(2)(CO)(4)(mu-sigma,sig ma(2e)-C=C=CR1R2)] transition state. Extended Huckel MO calculations h ave been carried out on the model [Cp(2)Mo(2)(CO)(4)(mu,eta(2)-C=C=CH2 )]. The calculated charges of the allenylidene carbon atoms suggest th at the electrophilic attacks are under charge control, while the nucle ophilic attacks are rather under orbital control.