The Collaborative International Dictionary
Metalammonium
Metalammonium \Met`al*am*mo"ni*um\, n. [Metal + ammonium.] (Chem.) A hypothetical radical derived from ammonium by the substitution of metallic atoms in place of hydrogen.
Wiktionary
metalammonium
n. (context chemistry English) A hypothetical radical derived from ammonium by the substitution of metallic atoms in place of hydrogen.