Substitution of Unidentate Ligands

Costanzo, A. Giuffrida, G. Guglielmo, and V. Richevuto, Inorg. Chim. Acta, 1979, 33, 29. 

Bromide substitution in the sterically hindered complexes [Pd(Et4dien)XCN] and [Pd(MeEt4dien)XCN]+ (X=S or Se) follows the usual two-term rate law in methanol solution, whereas in the case of the nitrogen-bonded linkage isomers only the solvolytic path is observable.  

Previous kinetic studies of palladium complexes containing sulphide ligands have dealt mainly with relatively inert systems with chelating or sterically hindered sulphides. The replacement of a variety of organic sulphides R SR (where R and R denote alkyl or phenyl groups) with pyridine and substituted pyridines according to reaction (14), 

Analysis of the results shows that there are very small differences in the discriminating ability of one complex against the various entering amines, whereas, when the reactivity of the series of complexes towards a given amine is compared, there is a marked decrease in rate when the donor ability of the sulphur atom is increased i.e. the lability decreases when the bond strength increases.  

In an effort to elucidate the influence of the non-reacting chelate ligands terpyridyl (terpy) and 3-azapentane-l,5-diamine (3-NHpd) on the reactivity of square-planar palladium complexes, the kinetics for the substitution reaction (15) 

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Reactions in Non-aqueous Solvents.—By studying the kinetics of replacement of water in [Fe(CN)5(OH2)] with a series of unidentate ligands in ethylene glycol containing > 0.055 mol 1 water, ASperger and co-workers have been able to distinguish between h and D mechanisms for the general substitution reaction... 

The exchange proreeds in three steps. The first step is substitution of the free ligand at one end of the chelate to give the intermediate I(fca). The second is intramolecular proton transfer between the unidentate ligand in l(kb). The third is the reverse of the first (k a). Consequently, application of the steady-state approximation to the intermediate 1, whose concentration is reasonably assumed to be very low, provides... 

Most studies have again been concerned with replacment of one unidentate ligand by another, and the rules and patterns of behavior that have evolved are based mainly on this simple type of substitution reaction. Consider the substitution scheme... 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

Several monosubstituted derivatives have been made in a similar manner, except that a unidentate ligand is used in place of the carbon monoxide, namely NH3, N2H4 (88) C2H4 (75) PPh3, AsPh3, CH3CN (231). Alternatively, substituted monooleiin derivatives have been synthesized by protonation of the metal a-allyl 7r-cyclopentadienyl tricarbonyl (55, 112). 

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