Metal Electron Delocalization in 3d Complexes Estimated by Positronium Spin Exchange Reactions

Abstract

Positronium, Ps, is the bound state between an electron and its antiparticle: the positron. Ps atoms may exist in two states: the ortho and para states: in the former the spins of the two particles are parallel (o-Ps) and antiparallel in the latter (p-Ps). The two types of Ps atoms may be experimentally distinguished each other. By interacting with paramagnetic compounds, such as many 3d complexes, the two types of Ps atoms may undergo the conversion reaction, CR, which consists in the conversion of o-Ps into p-Ps atoms and viceversa. Present paper is the final account of a series concerning the CR of Ps atoms promoted by ca 70 complexes of V(II),Cr(II),Cr(III),Mn(II),Co(II),Ni(II) ions. The starting points of these researches was that the rate constants, kCR of the CR should depend on density of unpaired 3d electrons at the periphery of the complexes. In particular it was hypothesized that the kCR should be linearly correlated with the metal electron delocalization caused by the ligands as it is described by the ratio beta between the inter-electronic repulsion parameter in 3d complexes, B, and that in the free ions, Bo. The beta values were estimated by the empirical equation beta=1-t h, where t and h are constants characteristic of the ion and ligand, respectively. The rate constants,kCR, of the reactions, were measured at 278, 288, 298 and 308 K. The analysis of the data, limited in a previous review to the experiments carried out at 298 K, is here extended to the other three temperatures in order to ascertain if the t and h values proposed are indipendent of T, as it is expected. The relationship between the correlations for complexes of the same ion J in high and low spin configurations is also taken into consideration. An experimental equation was derived for evaluating kCR as a function of T and beta. The analysis of the data shows that experimental and back-calculated kCR are in nice agreement between themselves, i.e. the mean-square error of the discrepancies between the values of experimental and back-calculated rate constants are 1.3-1.4 times the mean-square error of the experimental data. This supports the statements advanced in order to explain the relationship between kCR and the chemical constitution of 3d complexes. Reciprocally, the statement that the kCR measurements constitute a new method for testing the metal electron delocalization in 3d complexes, gains further supports. A short account on the o-Ps into p-Ps conversion reactions, their mechanisms and kinetics is also given.

Keywords: Inorganic Chemistry > Spectroscopy, inorgchem/0007001