Spectroscopy, solvation and reactivity of transition metal complexes with Pi-acceptor ligands
Information from solubility, spectroscopy and kinetic studies has been used to characterize solvation for three groups of complexes. These three groups include metal-d6 diimine complexes, metallocenes (M(Cp)2Cl2 where M = Ti(IV), V(IV) and Zr(IV)), and binuclear complexes involving d6-metal ions with n-acceptor bridging ligands. The discussion of the first group deals mainly with Mo(0), and also with a few Mn(I) analogues. Solubilities, from which transfer chemical potentials are derived, have been measured in various pure and mixed solvents. Information about charge transfer spectra, carbonyl stretching frequencies and n.m.r chemical shifts (several nuclei) are reported. The results clearly indicate solvent dependence. Kinetics of substitution reactions involving formation from Mo ( CO)5 (4CNpy) or from Mn(CO)5Br and substitution of Mo(CO)4(diimine) complexes have been studied in various solvents. The solvent sensitivities of reactivity of the latter process are analysed, based on initial state-transition state contributions for these reactions of well-established mechanism. Pressure effects on reactivity, and on charge transfer spectra of complexes in this first group, have been investigated. Kinetics of reaction between M(Cp)2Cl2 with NCS- in acetonitrile have been used to assess the solvation of the complexes. The rate law for this reaction contains first-order and second-order terms. The effect of addition of water or toluene to the main medium on the rate constant is reported. The binuclear complexes of d6 -metal ions with Tt-acceptor bridging ligands, like mononuclear complexes of the first group, also exhibit charge transfer spectra. Absorption occurs at rather lower energy and shows higher solvent sensitivities than mononuclear analogues. The majority of the complexes are relatively unstable. The rate of decomposition of the pyrazine carboxylate bridged [(en)2Co(pyzCOO)Fe(CN)5] anion, which follows intramolecular electron transfer, has been measured in binary aqueous solvent media. Its rate constant is markedly solvent dependent; no correlation with Marcus-Hush theory is observed.