A new compound (NH 4 ) 2 [Ni(H 2 O) 5 (NH 3 )] 2 [V 10 O 28 ]·4H 2 O (1) containing a {V 10 O 28 } 6− anionic cluster and a novel complex cation, [Ni(H 2 O) 5 (NH 3 )] 2+ , has been synthesized and fully characterized by single crystal X-ray crystallography, spectroscopy and thermogravimetric analysis. The presence of the ammonia ligand in the complex cation in 1 was established unambiguously by X-ray crystallography and variable temperature (200–400°C) thermogravimetric analyses in combination with FTIR spectroscopic studies. The formation of the novel complex species {Ni(H 2 O) 5 (NH 3 )} 2+ during the synthesis of 1 can be rationalized in terms of ligand substitution involving {Ni(H 2 O) 6 } 2+ .