Substitutions for Dienes and Trienes

Substitution for polyhapto ligands occur in many cases by mechanisms that are similar to those for replacement of monodentate ligands. Thus, substitutions for polyhapto ligands in 18-electron octahedral complexes, like substitutions for CO in the 18-electron octahedral complexes presented in Section 5.4.2.1, often occur by competing dissociative and associative pathways with a two-term rate expression, = k + k  

Cr(CO) (butadiene) reacts with relatively strong dative ligands like P(OMe)j to form Cr(CO) [P(OMe)j]2 and free diene, as shown in Equation 5.43, and the rates of these reactions fit a two-term rate law consisting of one term for a first-order process in which the rate constant fcj corresponds to dissociation of one half of the diene ligand and a second, measurable lc2[P(OMe)3] term reflecting a second order process with some associative component. In contrast, the reaction of Cr(CO) (butadiene) with the more weakly nucleophilic 1,5-COD forms Cr(CO) (l,5-COD) and free butadiene by a single first-order pathway initiated by dissociation of half of the diene. 

If the number of olefins in the polyene is greater than two, then the number of steps in the overall displacement reaction is simply larger. For example, M(CO)3(l,3,5-cyclo-heptatriene) (wherein which M = Cr, Mo, and W) reacts with three equivalents of PhCN to form Mo(CO)3(NCPh)3 and 1,3,5-cycloheptatriene by a stepwise process. The rate law for reaction of Cr(CO)3(l,3,5-cycloheptatriene) is first order in complex and first order in benzonitrile. However, the reaction of Mo(CO)3(l,3,5-cycloheptatriene) is first order in complex and second order in benzonitrile. The kinetic data for reaction of the molybdenum complex have been rationalized by a stepwise pathway in which the first benzonitrile 

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