what are complex compounds

The addition compounds in which the some of constituntes ion or molecule loose their identity ,when dissolved in water they do not break up in indivisual ions.

Since they do not break up in their indivisual ions hence they are called complex compounds.

For example K4[Fe(CN)6] breaks into 4K^+  +[Fe(CN)6]^4-

It does not give the taste of Fe^2+ and CN^- ions.

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In chemistry, a coordination complex or metal complex, is a structure consisting of a central atom or ion (usually metallic), bonded to a surrounding array of molecules or anions (ligands, complexing agents).   The atom within a ligand that is directly bonded to the central atom or ion is called the donor atom. Polydentate (multiple bonded) ligands can form a chelate complex. A ligand donates at least one pair of electrons to the central atom/ion.

Compounds that contain a coordination complex are called coordination compounds. The central atom or ion, together with all ligands form the coordination sphere. 

Coordination refers to the "coordinate covalent bonds" (dipolar bonds) between the ligands and the central atom. Originally, a complex implied a reversible association of molecules, atoms, or ions through such weak chemical bonds. As applied to coordination chemistry, this meaning has evolved. Some metal complexes are formed virtually irreversibly and many are bound together by bonds that are quite strong.

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