Influence of coordination on N-H X hydrogen bonds. Part II. Zn(NH3)2Br2 and Ni(NH3)2X2 (X is Cl and Br)

Microcrystalline samples of Zn(NH₃)₂Br₂ and Ni(NH₃)₂X₂ (X is Cl⁻ and Br⁻) have been investigated from 100 to 293 K using X-ray diffraction and IR spectroscopy measurements (range 400–4000 cm⁻) performed with isotopically dilute (5% deuterated) samples. Values of Δν(ND)/ΔT for all compounds hint at the existence of hydrogen bonds. Zn(NH₃)₂Br₂ shows The dynamics of ammonia molecules even at 100 K, and no indications are apparent that dynamic disorder of ammonia molecules takes place in Ni(NH₃)₂X₂ (X is Cl⁻ and Br⁻). A comparison between octahedrally coordinated ammoniates Ni(NH₃)₆]Br₂, Ni(NH₃)₂Br₂ and Zn(NH₃)₆]Br₂ with tetrahedrally coordinated ones Zn(NH₃)₂Br₂] leads to the conclusion that the lower coordination number increases the strength of the hydrogen bonds. Because this effect is small, it is not possible to separate the influence of the type of coordinating ions for one coordination number from the influence of the coordination number itself.