Crystal structure of di-µ-hydroxo-trans-diaquo-bis[triamminecobalt(III)] tetranitrate dihydrate, and a possible mechanism for the formation of the cation

Abstract

The di-µ-hydroxo-bis[aquotriamminecobalt(III)]⁴⁺ cation is formed in acidic solution from the tri-µ-hydroxo-bis[triamminecobalt(III)]³⁺ complex. The crystal structure of the hydrated nitrate salt of the di-[µ-hydroxo-cation], [Co₂(OH)₂(H₂O)₂(NH₃)₆](NO₃)4,2H₂O, has been reinvestigated and refined to R 0·032 [space group P2₁/n, a= 9·444(2), b= 9·684(3), c= 10·736(3)Å, β= 90·1(2)°, Z= 2]. All hydrogen atom positions were found from a difference synthesis, which resulted in an unambiguous assignment of all hydrogen bonds, and enabled the H₂O and NH₃ groups to be distinguished. The water molecules co-ordinated to the Co-atoms are trans. The ligands in the Co³⁺ complex undergo rearrangement after breaking of the hydroxo-bridge, probably by formation of a reactive five-co-ordinate intermediate