Equilibrium constants have been obtained spectrophotometrically for the stepwise replacement by iodide of the chloride in trans-Rh en₂Cl₂⁺, and of the bromide in trans-Rh en₂Br₂⁺. At 90°K₁(Cl^–:I^–)= 7·0. Extrapolation to 90° of data obtained at lower temperatures gives K₂(Cl^–:I^–)= 2·2 and K₂(Br^–:I^–)= 1·9. The temperature-dependence of the equilibrium constants lead to the following enthalpy data. ΔH₁°(Cl^–:I^–)=–0·1 ± 0·9, ΔH₂°(Cl^–:I^–)=–6·2 ± 9·4, ΔH₁°(Br^–:I^–)=–2·6 ± 0·4, and ΔH₂°(Br^–:I^–)= 3·4 ± 0·3 kcal./mole.

From these, and other obtained previously, it is possible to obtain the relative enthalpies of bonding of the dihalogeno-complexes in aqueous solution. These are in the order di-iodo > iodobromo > iodochloro > dibromo > bromochloro > dichloro. The values are discussed in terms of class (a) or (b) character of these complexes and the extent to which this is affected by the nature of the halide in the trans-position.

The preparation and ultraviolet and visible absorption spectra of some new trans-bisethylenediaminediacidorhodium(III) complexes are reported.