One of a group of complex compounds formed by the coordination of ammonia molecules with metal ions and, in a few instances, such as calcium, strontium, and barium, with metal atoms. Some typical examples of ammines include [Co(NH3)6]Cl2 (rose), [Cu(NH3)4]Cl2 (blue), [Cr(NH3)6]Cl3 (yellow), [Cr(NH3)4Cl2]Cl (cis is violet, trans is green), [Ni(NH3)6]Cl2 (blue), [Pt(NH3)4]Cl2·H2O (colorless), and [Hg(NH3)2]Br2 (colorless). Although these ammines are formally analogous to many salt hydrates, the general characteristics of the group of ammines differ considerably from those of the hydrates. For example, hydrated Co(III) salts are strong oxidizing agents whereas Co(II) ammines are strong reducing agents. The ammines of principal interest are those of the transition metals and of the zinc family, but even here there is wide variation in stability or rate of decomposition. For example, iron ammines are unstable in aqueous solution; Cu(II) and Co(II) ammines exist in aqueous solution but are decomposed by aqueous acids; Co(III) and Pt(IV) ammines can be recrystallized from strong acids. Ammines are prepared by treating aqueous solutions of the metal salt with ammonia or, in some instances, by the action of dry gaseous or liquid ammonia on the anhydrous salt. These, and similar, differences have motivated much theoretical study of the bonding in such compounds. See also: Ammonia; Chemical bonding; Coordination chemistry