NMR study of the nature of the second coordination sphere in aqueous solutions of ruthenium(III) hexaammine

A diamagnetic shift and a longitudinal relaxation rate of the water protons as well as a paramagnetic shift of the ¹⁷O nuclei of water were measured in acidic aqueous solutions of the Ru(NH₃)₆³⁺ ion. Possible effects for the origin of the diamagnetic protons shift were examined. It was concluded that the shift can only be attributed to water molecules in the second coordination sphere of the Ru(NH₃)₆³⁺ ion. The distance of closest approach of the water molecules from the complex ion was evaluated from the longitudinal relaxation rates as 3.9 Å by using a translational diffusion model. An effort was made to establish the position of the water molecules in the second coordination sphere by the distance of closest approach and the diamagnetic water protons shift. It was discussed that none of the mechanisms for delocalization of spin density can explain the experimental fact of the diamagnetic water protons shift. It was suggested that the diamagnetic shift originated from a through-space pseudocontact interaction of spin density on the neighboring ammonia molecules. Calculations were done to show that water molecules occupying the centers of the faces of the complex ion octahedron might experience a diamagnetic pseudocontact shift as observed experimentally.