Optical rotatory power of co-ordination compounds. Part V. The outer-sphere complexes of the tris-diamine–cobalt(III) complex ions

Abstract

The stability constants of the outer-sphere complexes ML_n, n= 1–4, formed by the (+)-trisethylenediamine-cobalt(III) ion (M) with selenite (L) have been determined from circular-dichroism measurements in aqueous solution at constant ionic strength, and from the formation constants the circular-dichroism spectra of the individual complexes ML_n have been obtained. From similar measurements and known stability constants, the circular-dichroism spectra of the outer-sphere complexes ML_n have been derived for L = thiosulphate, n= 1–4, and L = ferrocyanide, n= 1 or 2. The results show that the rotational strengths of the A₂ and the E_a components of the octahedral T_1g transition of cobalt(III) are enhanced and diminished, respectively, in the complexes ML_n with n odd, and that the circular-dichroism spectrum reverts towards that of the free complex ion M in the corresponding complex ML_n+1. Trends in the relative stabilities of the outer-sphere complexes and in the intensity of the interionic charge-transfer absorption and circular-dichroism bands indicate that dispersion and charge-transfer forces become progressively more important in outer-sphere co-ordination as the overall charge of the complex ML_n becomes more negative. In the group D₃ of the free complex ion the charge-transfer transition has E symmetry in the complexes ML_n with n odd and has components with A₂ and E symmetry if n is even.