(i) Coordination entity: A complex compound that constitutes a central metal (atom or ion) linked with a fixed number of ions or molecules. For example,
[Ni(CO)4], [COCl3(NH3)3], [PtCl2(NH3)2], [Fe(CN)6]4- etc.
(ii) Central atom/ion: The atom or ion to which a fixed number of ions/groups are bound in a certain geometrical arrangement around it. Since it accepts a lone pair of electrons for the formation of coordinate bond, it is also referred to as Lewis acids. For example, Fe3+ and Ni2+ are the central ions in the coordination compounds [Fe(CN)6]3- and [NiCl2(H2O)4] respectively.
(iii) Ligands: The atoms, ions or molecules which donate a pair of electrons to the metal atom to form a coordinate bond, are called ligands. For example,
NH3, H2O, Cl-, CN-, Br-, C2O42-, etc.
Depending on the number of donor atoms, a ligand can be of following types:
(a) Unidentate or Monodentale ligand: It contains only one donor atom. For example, H2O and NH3 in which O and N are the donor atoms which bind with metal atom or ion.
(b) Didentate or Bidentate ligand: When a ligand has
two donor atoms, for example, ethane-1,2-diamine (H2NCH2CH2NH2), in which the two nitrogen atoms of the amino group act as donor atoms.
(c) Polydentate or Multidentate ligand: When several donor atoms are present in a single ligand, for example EDTA4- (Ethylenediaminetetraacetate), is an important hexadenate ligand which can bind through two nitrogen and four oxygen donor atoms to a central metal ion.
(iv) Chelate ligand: A di- or polydentate ligand is said to be a chelate ligand when it uses its two or more donor atoms to bind a single metal ion. The number of such ligating groups is called the denticity of the ligand.
A complex compound in which the donor atoms are attached to the metal so that the metal becomes a part of the heterocyclic ring, is called chelate complex.
(v) Coordination number (CN): The number of unidentate ligands directly bonded to the central metal atom/ion is known as the coordination number of that metal ion/atom.
For example, in the complex ions, [Ag(NH3)2]2+, [Zn(CN)4]2- and [Ni(NH3)6]2+
the coordination number of Ag, Zn and Ni are 2, 4 and 6 respectively.
When the bonded ligands are didentate the coordination number is double the number of ligands because the number of bonds linked to the central metal becomes double. For example, the coordination number of Fe in [Fe(C2O4)3]3- is 6, because C2O42- is a didentate ligand.
(vi) Coordination polyhedron: It describes the spatial arrangement of the ligand atoms which are directly attached to the central atom/ion. For example, the coordination polyhedra of [Ni(CO)4], [Co(NH3)6]3+ and [PtCl]2- are tetrahedral, octahedral and square planar respectively.
(vii) Coordination sphere: The coordination complex which constitutes the central atom/ion and the ligands, are represented in a square bracket, collectively termed as coordination sphere. The ionisable groups are written outside the bracket, called counter ions. For example, in the complex K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4- and the counter ion is K+.
(viii) Homoleptic and Heteroleptic complexes: The complex in which metal ion is bound to only one kind of ligands is called the homoleptic complex and when more than one kind of ligands are bound to the metal, it is called heteroleptic complex. For example,
Homoleptic complex : [Co(NH3)6]3+, [Fe(CN)6]4-, etc
Hoteroleptic complex : [CoCl2(NH3)4]+, [PtCl(NH3)5]3+, etc
(ix) Charge on a complex ion: The charge carried by a complex ion is the algebraic sum of charges carried by the central metal ion and the coordinated groups or ions. For example,
[Fe(CN)6]3-Fe3+ = +3 charge; 6CN- = 6 × (-1) = -6 charge
Thus charge on Fe(CN)6 = +3 - 6 = -3.
(x) Types of complex ion:
(a) Cationic complex : [Cr(H2O)4Cl2]+
(b) Anionic complex : [Fe(CN)6]3-
(c) Neutral complex : [Ni(NH3)4Cl2]