This is because it contains the cyanide ion, CN-, which is a polyatomic anion. An anion is a negatively charged ion, and a cation is a positively charged ion. When a metal atom reacts with a non-metal atom, the metal atom will lose electrons to the non-metal atom, resulting in the formation of a cation and an anion. In the case of Cr(CN)3, the chromium atom loses three electrons to the three cyanide ions, resulting in the formation of the Cr3+ cation and the three CN- anions. These ions then combine to form the ionic compound Cr(CN)3.