12.6.2: Evidence for Associative Reactions

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The reactions of Pt(II) complexes have been thoroughly investigated, and there are several lines of experimental evidence that have lead to the general conclusion that square planar complexes react through associative mechanisms. The experimental evidence is presented here.

The entering group

In the reaction of \(\ce{trans-[PtL2Cl2]}\), the rate of reaction depends on the identity of the incoming ligand Y.

Figure \(\PageIndex{1}\): Substitution reaction of \(\ce{trans-[PtL2Cl2]}\). (CC-BY-SA; KAthryn Haas)

For example, the rate of the reaction shown above depends heavily on the ligand, Y, as follows:

\[ \text{most kinetically labile }\ce{PR_3 > CN^- > SCN^- > I^- > Br^- > N3^- > NO2^- > py > NH3 ~ Cl^- > CH3OH } \text{ most kinetically inert} \nonumber \]

In general, ligands that would form the strongest bonds with Pt(II) due to their soft character or \(\pi\)-binding ability react fastest. The rate constants in the above series range by nearly ten orders of magnitude. This huge range of rate constants as a factor of Y demonstrates suggests that M-Y bond formation is part of the rate limiting step. This is evidence for rate-determining association \(A\) or \(I_a\).

The leaving group

Since the leaving group occupies a similar position in the transition state as the entering group, we should expect the leaving group to have a similar effect on the rate constants as was found for Y. In fact, this is true. For the reaction of \(\ce{[Pt(diene)X]^+}\) with pyridine, the follow trend was observed by varying the leaving group, X:

\[ \text{most kinetically labile }\ce{NO3^- > Cl^- > Br^- > I^- > N3^- > SCN^- > NO2^- > CN^-} \text{ most kinetically inert} \nonumber \]

The rate of reaction follows an order that is nearly opposite as that observed for Y: The softer the ligand, the slower the dissociation. The rate constants in this series span a range of approximately five orders of magnitude, illustrating that breaking of the M-X bond is part of the rate-determining step. This is evidence for a mechanism involving associative interchange character (\(I_a\) or \(I_d\). The importance of the leaving group somewhat depends on the identity of the entering group, and vice-versus.

The trans ligand

The trans ligand also occupies a similar position to X and Y in the transition state. So, as you might expect, it too has influence on the rate of reaction. In general, the rate of substitution of X in Pt(II) complexes general follows the order given below when T is varied as:

\[ \text{M-X most kinetically labile }\ce{CN^- ~ CO ~ C2H4 > PH3 ~ SH2 > NO2^- > I^- > Br^- > py ~ NH3 > OH^- > H2O } \text{ M-X most kinetically inert} \nonumber \]

The effect of the trans ligand deserves a more through discussion, and will be addressed in the next section. It is mentioned here because it is evidence for the trigonal bipyramidal intermediate that was described earlier for associative substitution in square planar metal complexes.