question_answer 13)

 Explain with two examples each of the following: Coordination entity, ligands, coordination number, coordination polyhedron, homoleptic and heteroleptic complex.

Answer:

(i) Co-ordination Entity (complex): A co-ordination entity constitutes a central atom/ion (usually for a metal) to which fixed no. of other atoms or groups (called ligands) are attached. It may be neutral or charged e.g. [Co (NH3)6]3+, [Cr(en)2Cl2]+. (ii) Ligand: A molecule or ion bound to the central atom/ion in the co-ordination entity is called ligand. It may be neutral, positively or negatively charged e.g. H2O, CN? etc. (iii) Co-ordination Number: The total number of co-ordination bonds formed by the ligands in the complex is called co-ordination number. e.g. [Ag(CN2)]? [Co (NH3)Cl3]    [Co(eb)3]2+ CN = 2         CN = 6            CN = 6 (iv) Co-ordination polyhedron: The spatial arrangement of the ligands attached to the central metal atom/ion in the co-ordination sphere or complex entity is called co-ordination polyhedron. The arrangement in fact, determines the geometry of the complex which may be tetrahedral, square planar, pyramidal, trigonalbipyramidal, octahedral etc. depending upon the number of the ligating groups present. Here M stand for the metal atom/ion while L indicates a monodenate ligand. (v) Homoleptic and heteroleptic complexes: Complexes in which the metal atom or ion is linked to only one type of ligands are called homoleptic complexes, e.g., [Co(NH3)6]3+ and [Fe(CN)6]4? etc. The complexes in which the metal atom or ion is linked to more than one kind of ligands are called heteroleptic complexes, e.g., [Co(NH3)4 Cl2]+ and [Cr(en)2Cl2]+ etc.