Sundry studies in the chemistry of coordination compounds

Abstract

Part I: Nickel-Thiourea Complexes The infra-red spectra of nickel-thiourea complexes have been compared with the spectrum of thiourea and changes in these spectra have been associated with the coordination of the thiourea. All compounds investigated have shown IR spectral shifts consistent with coordination of the thiourea through sulphur. Criteria for distinguishing monocoordinate and bridging thioureas have been put forward, but π-coordination of thiourea appears to be indistinguishable from lone-pair coordination by IR spectral measurements. Nitrogen substituted thioureas also show IR shifts on coordination but these shifts are not as reliable indicators of the mode of coordination as for unsubstituted thiourea because they are frequently too small to measure reliably. The IR spectra of various mono, di and tetra substituted have been alalysed utilizing all the available literature data as well as our own data. Hydrogen bonding and Fermi Resonance have been illustrated in the IR spectrum of 1,3 di iso propyl thiourea. A model has been proposed, and discussed, to explain the structure and properties of nickel-thiourea complexes. The basic unit is a planar Nitu42+ species stabilized by a “ring MO” similar to that in Ni(CN)42-. Coordination in the 5 and 6 positions may or may not occur, and may involve further thiourea molecules, solvent molecules or anions depending on the relative coordinating powers of the species present. With very strongly coordinating anions e.g. halides, the Nitu42+ unit may be broken and tetrahedral Nitu2X2 species result. Part II The crystal field method of estimating activation energies, used by the present author for trisoxalato complexes (J.Chem.Soc. 2147 (1964)), has been discussed in more detail, and estimates made of the magnitudes of energy terms neglected in the simple approach. These terms include: gross electrostatic repulsions, spherically symmetrical crystal field terms, hydration energies, and bond-making and –breaking terms. Further, an alternative to the “one-ended dissociation” model of the reactivity of coordinated oxalate has been proposed in which the π-bond of the carbonyl group is coordinated to the central metal ion. Such a model is consistent with the known reactions of the trisoxalates of chromium(III) and cobalt(III). The catalysis of the aquation and racemisation reactions of trisoxalato chromium(III) by various metal ions arises because the catalyzing ion reorientates the solvent sheath about the complex, rather than causing a bond-weakening of the chromium-oxalate bond by electron withdrawal. All the available data are consistent with this new hypothesis. In the final two chapters are reported some calculations on transition metal and other ions. First it is shown that the correlation energy for 3dn ions can be estimated relatively easily by using the parameters r< and r> in the expression for the Slater-Condon parameters. Such a method leads to a reduction of the calculated Hartree-Fock spectroscopic term separations and brings them closer to those observed. Secondly it is shown that good values of Ionisation Potentials, Electron Affinities and Electronegativities can be obtained by simply using the arithmetic means of the values calculated by two standard methods viz. energy expectation value differences and Koopmans’ Theorem.