Redetermination of the structure of nitrosylpenta-amminecobalt(III) dichloride

Abstract

The structure of nitrosylpenta-amminecobalt(III) dichloride, [Co(NO)(NH₃)₅]Cl₂, has been redetermined by use of three-dimensional X-ray data collected by counter methods. The compound crystallizes in the orthorhombic space group D_2h¹⁷–Cmcm, with a=c= 10·459(7) and b= 8·753(6)Å. Each crystal grows as an intimate twin, one individual of a twin being related to the other by the interchange of the a and c axes. The hkl reflection from one individual is overlapped with the kh reflection from the other such that the resulting diffraction pattern has pseudotetragonal symmetry. Refinement was effected by modifying our least-squares program to add the intensity contributions from the two individuals. In addition, a parameter describing the degree of twinning was refined, and the disordered nitrosyl ligand was treated as a group. The final R on F² is 4·2% for the 587 independent reflections having F² > 3σ(F²). Previous structural determinations, based on the non-overlapped reflections observed by photographic means, led to a linear Co–N–O group. This geometry would imply the unlikely co-ordination Co¹–NO⁺. Our Co–N–O bond angle of 119·0(9)° indicates the co-ordination Co^III–NO^–. The long Co–NO distance of 1·871(6)Å and the long Co–NH₃(trans) distance, 2·220(4)Å, support this interpretation.