Coordination Compounds

Co-ordination compounds: In a complex ion, the central metal ion is surrounded by a number of negative ions or the neutral molecules (known as ligands) which are linked by co-ordinate or dative bonds. Therefore, the complex compounds are often called co-ordination compounds.

Co-ordination number (C.N.): The total number of monovalent (monodentate) ligands or the total number of donor atoms co-ordinated to the central metal atom or ion in a complex, is called co-ordination number of the central metal ion or atom.

For example, in the complex, K3[Al(C2O4)3], the co-ordination number of aluminium ion is six because there are six donor oxygen atoms of three oxalate ions co-ordinated to aluminium ion.

Co-ordination sphere: The central atom or ion along with the co-ordinated ligand ions or molecules in a complex ion, are enclosed in square brackets [   ]; it is collectively called co-ordination sphere. For example in the complex [CO(NH3)5Br]SO4, the five ammonia molecules and one bromide are co-ordinated to central cobalt ion. They are in the co-ordination sphere.