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 MLn, 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 MLn have been obtained. From similar measurements and known stability constants, the circular-dichroism spectra of the outer-sphere complexes MLn 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 Ea components of the octahedral T1g transition of cobalt(III) are enhanced and diminished, respectively, in the complexes MLn with n odd, and that the circular-dichroism spectrum reverts towards that of the free complex ion M in the corresponding complex MLn+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 MLn becomes more negative. In the group D3 of the free complex ion the charge-transfer transition has E symmetry in the complexes MLn with n odd and has components with A2 and E symmetry if n is even.