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Studies in metal-ligand bonding.

Norris, Paul Richard. (1996) Studies in metal-ligand bonding. Doctoral thesis, University of Surrey (United Kingdom)..

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The work in this thesis centres around direct equilibrium studies on the only two "soft" metal ions of the first row transition metals, low-spin d6 FeII and CuI, in relation to the linear free energy relationship given below. logK = a.pK + b. It is shown that solid Na3[Fe(CN)5(NH3)], which hydrolyses rapidly in solution to give the unstable [Fe(CN)5(OH2)]3- complex, can be dissolved in 0.1 mol dm-3 aqueous acetonitrile to give the much more stable complex [Fe(CN)5(MeCN)]3- (log K MeCN = 2.6 at 25°C, I= 0.1 mol dm-3). This more stable complex can then be used as the starting point for determining equilibrium constants for ligand substitution (of MeCN, and thus indirectly of H2O) on the "soft" low-spin d6 FeII ion under equilibrium conditions and over a wide range of pH (3 to 13 at least). Over 30 equilibrium constants are determined by uv-vis spectrophotometry for a number of nitrogenous bases (from four different families; RNH2, imidazoles, pyridines and RCN), with additional data determined for a series of sulphur containing ligands, which show that RS-, RCOS-, thiourea and RSH all form a single family. The linear free energy relationship is verified for each family of ligands studied, and in every case the value of a is shown to be ca 0, i.e. the equilibrium constant is apparently independent of the ligand basicity in both families of a-only and potential pi-acceptor ligands. Similar data are obtained for the water soluble porphyrin, FeII MP-8 in 20% aqueous methanol, which also show a ~ 0 for three families (RNH2, imidazole and pyridines). This establishes that a near zero value of a to be characteristic of the low-spin FeII ion. The alpha-effect and the steric effect are also studied with both FeII complexes. A method is also described in which the values of a can be semi-quantitatively determined for the "soft" CuI ion, by equilibration of solid CuCl in a 1 mol dm-3 aqueous solution of the ligand for one hour (25°C, pH 12, I= 0.1 mol dm-3) under hydrogen. Simple conversion to the CuII(EDTA) complex, after filtration, provides a suitable method for determining the concentration of copper solubilised after equilibration. Values of log [Cu]N are determined for about 15 nitrogenous bases (as a means of indirectly comparing the N-bases on the same basis), with a calpha 0 found for the three families studied (RNH2, imidazoles and pyridines). A coordination number of 2 is found for imidazole, and assumed throughout. This work provides the first detailed study of either of the "soft" first row transition metal ions, demonstrating that a is calpha 0 in both cases (cf other d6 and d10 ions). Data are related to published work on various d6 and d10 ions, and a remarkable similarity between the behaviour of the two is demonstrated. The data are, in turn, inversely correlated to the known gas-phase d-s promotion energies.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors :
Norris, Paul Richard.
Date : 1996
Depositing User : EPrints Services
Date Deposited : 09 Nov 2017 12:16
Last Modified : 09 Nov 2017 14:45

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