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Mechanisms of Ligand Association in Complexes of the Type RML3+X- (M = Ni, Pd, Pt). 11. The Mechanisms of the Reactions HML3+ L HML4+ (L = P(C2H5)3)
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A. D. English,* P. Meakin,* and J. P. Jesson* Contribution No. 2262 from the Central Research and Development Department, Experimental Station, E.I. du Pont de Nemours and Company, Wilmington. Delaware 19898. Received April 18, I975
Abstract: The reaction HML3+ + L F= HML4+ (M = Ni, Pd, Pt; L = P(C2H5)3) has been studied in detail using 31P{lH] and 'H NMR. The behavior of the NMR line shapes has been analyzed using general computer programs which include both inter- and intramolecular exchange effects. In addition to the kl and k-1 processes, rate processes corresponding to intramolecular rearrangement in both HML3+ and HML4+ species have been quantitatively analyzed. Detailed mechanistic information, such as the mode of ligand attack and the site occupied by the attacking ligand in the five-coordinate intermediate, has been developed. Direct spectroscopic evidence is presented for the five-coordinate intermediate for all three metals; the equilibrium lies to the right for Ni and very far to the left for Pt (acetone solution, -90 to +60°). The broad mechanistic details are essentially the same for all three metals: an HML3+ complex of CzUsymmetry undergoes association with a free ligand molecule to give an HML4+ intermediate of C3, symmetry (strongly distorted trigonal bipyramidal). The HML4+ species, in the case of Pt, is of such low concentration over the temperature range -90 to +60° that its presence cannot be detected by direct spectroscopic techniques; very low temperature (-140') NMR studies allow HPtL4+ to be observed directly; the studies in the -90' to +30° range show unambiguously that the attacking ligand ends in a position in HPtL4+ symmetry equivalent with the two trans ligands in HPtL3+ (i.e., in what would be the equatorial plane of the "trigonal bipyramid"). For Pt, the ligand dissociation from HML4+ ( k - 1 ) is rapid relative to the rate of intramolecular rearrangement in HML4+ ( k , ) , Le., k-1 >> k,, for Pd k - I = 102k,, and for Ni k-l
