Question -
Answer -
(i) Coordinationentity:
A coordination entity is an electricallycharged radical or species carrying a positive or negative charge. In acoordination entity, the central atom or ion is surrounded by a suitable numberof neutral molecules or negative ions ( called ligands). For example:
= cationic complex
= anionic complex
= neutral complex
(ii) Ligands
The neutral moleculesor negatively charged ions that surround the metal atom in a coordinationentity or a coordinal complex are known as ligands. For example,
, Cl−, −OH.Ligands are usually polar in nature and possess at least one unshared pair ofvalence electrons.
(iii) Coordinationnumber:
The total number of ligands (either neutralmolecules or negative ions) that get attached to the central metal atom in thecoordination sphere is called the coordination number of the central metalatom. It is also referred to as its ligancy.
For example:
(a)In the complex, K2[PtCl6], there as sixchloride ions attached to Pt in the coordinate sphere. Therefore, thecoordination number of Pt is 6.
(b)Similarly, in the complex [Ni(NH3)4]Cl2,the coordination number of the central atom (Ni) is 4.
(vi) Coordinationpolyhedron:
Coordinationpolyhedrons about the central atom can be defined as the spatial arrangement ofthe ligands that are directly attached to the central metal ion in thecoordination sphere. For example:
(a)
(b) Tetrahedral
(v) Homolepticcomplexes:
These are those complexes in which the metal ion is bound to only onekind of a donor group. For eg: 
etc.
(vi) Heterolepticcomplexes:
Heteroleptic complexes are those complexeswhere the central metal ion is bound to more than one type of a donor group.
For e.g.: