Solid complexes of the type [UX₂L₄(ClO₄)₂][X = Cl or Br; L = triphenylphosphine oxide (tppo), tris(dimethylamino)phosphine oxide (tdpo), tris(pyrrolidin-1-yl)phosphine oxide (tpyrpo) or triphenylarsine oxide (tpao)] were isolated and characterized. All formed six-co-ordinated doubly charged cationic complexes in polar non-aqueous solvents. The major bands around 1900 nm in the electronic spectra were shifted towards higher energy as the donor strength of L increased and could be used to identify the different species. Spectrophotometric titrations of solutions of [UX₂(ClO₄)₂] with L in acetone indicated disproportionation, resulting in the formation of [UX₄L₂] and [U(ClO₄)₄L_n] moieties with n varying from 0 to 5. Differences between the chloro and bromo systems were identified. Spectral studies on solutions of [U(ClO₄)₄(tpao)_n] when n < 2 suggested the presence of both mono- and bi-dentate perchlorato groups. The complex [U(ClO₄)₄(tpao)₄], which has unusual perchlorate co-ordination, was isolated as a surprisingly stable solid. The structures of the isomorphous pair [UX₂(tpyrpo)₄][BPh₄]₂(X = Br or I) were determined by X-ray crystallography. The compounds are monoclinic, space group P2₁/c, with a= 28.120(5), b= 15.998(4), c= 22.010(5)Å and β= 98.01(3) for the bromide and a= 28.172(5), b= 16.007(6), c= 22.059(5)Å and β= 97.91(3) for the iodide. The structural details of the compounds were comparable. The uranium atoms have trans-octahedral co-ordination spheres with mean bond lengths U–Br 2.775, U–I 2.980, U–O 2.230 Å(bromide) and 2.216 Å(iodide).