Mechanism of triphenylphosphine dissociation from Cis-( bis-triphenylphosphine) tetracarbonyl molybdenum(0)

The chemistry of the metal carbonyl complex cis(eta (1)-PPh3)(2)Mo(CO)(4) w as revisited. This complex undergoes a spontaneous dissociation of PPh3 to produce the complex (eta (1)-PPh3)Mo(CO)(5). The reaction is first-order wi th respect to cis-(eta (1)-PPh3)(2)Mo(CO)(4). The reaction was studied unde r pseudo-first order conditions where [PPh3] >> [cis-(eta (1)-PPh3)(2)Mo(CO )(4)]. Under flooding conditions the rate constant values decrease with inc reased concentration of PPh3. The activation parameters' values for the rea ction in various solvents and the observation that the values of the rate c onstant depend on the solvent's coordinating ability suggest an interchange mechanism. In the proposed mechanism, the dissociation of PPh3 from cis-(e ta (1)-PPh3)(2)Mo(CO)(4), is solvent-assisted, producing the intermediate s pecies cis-(eta (1)-PPh3)(solv)Mo(CO)(4) (solv = solvent). In the presence of a relatively high PPh3 molar concentration (ca. 0.2 M or higher) dissoci ation PPh3 from the parent complex was not observed and instead an extremel y slow cis to tuans isomerization was observed.