Coordination compounds are those addition molecular compounds which retain their identity in solid state as well as in dissolved state.
In these compounds. the central metal atom or ion is linked by ions or molecules with coordinate bonds. e.g., Potassium ferrocyanide, K4 [Fe(CN)6].
These are the addition molecular compounds which are stable in solid state but dissociate into constituent ions in the solution. e.g., Mohr’S salt, [FeSO4.(NH4)2So4.6H2O] get dissociate into Fe(2 +ve ions), NH4(1 +ve ion) and SO4(2 -ve ions).
Terms Related to Coordination Compounds-
1. Complex ion or Coordination Entity-
It is an electrically charged species in which central metal atom or ion is surrounded by number of ions or neutral molecules.
(i) Cationic complex entity– It is the complex ion which carries positive charge. e.g., [Pt(NH3)4]2+
(ii) Anionic complex -It is the complex ion which carries negative charge. e.g., [Fe(CN)6].
2. Central Atom or Ion-
The atom or ion to which a fixed number of ions or groups are bound is .ned central atom or ion. It is also referred as Lewis acid. e.g., in (NiCI2(H2O)4]. Ni is central metal atom. It is generally transition element or inner-transition element.
3. Ligands –
Ligands is electron donating species (ions or molecules) bound to the Central atom in the coordination entity.
These may be charged or neutral. LIgands are of the following types :
(i) Unidentate- It is a ligand, which has one donor site, i.e., the ligand bound to a metal ion through a single donor site. e.g., H2O, NH3, etc.
(ii) Didentate– It is the ligand. which have two donor sites.
(iii) Polydentate- It is the ligand, which have several donor sites. e.g., [EDTA]4(-ve ions) is hexadentate ligand.
(iv) Ambidentate ligands- These are the monodentate ligands which can ligate through two different sites, e.g., NO(2 -ve ions), SCN(-ve ion), etc.
(v) Chelating ligands- Di or polydentate ligands cause cyclisation around the metal atom which are known as chelate IS , Such ligands uses two or more donor atoms to bind a single metal ion and are known as chelating ligands.
More the number of chelate rings, more is the stability of complex. The stabilisation of coordination compounds due to chelation is known as chelate effect. π – acid ligands are those ligands which can form π – bond and n-bond by accepting an appreciable amount of 1t electron density from metal atom; to empty π or π – orbitals.
4. Coordination Number-
It is defined as the number of coordinate bonds formed by central metal atom, with the ligands.e.g., in [PtCI6]2(-ve ions), Pt has coordination number 6. In case of monodentate ligands, Coordination number = number of ligands. In polydentate ligands, Coordination number = number of ligands * denticity.
5. Coordination Sphere-
The central ion and the ligands attached to it are enclosed in square bracket which is known as coordination sphere. The ionisable group written outside the bracket is known as counter ions.
6. Coordination Polyhedron-
The spatial arrangement of the ligands which are directly attached to the central atom or ion, is called coordination polyhedron around the central atom or ion.
7.Oxidation Number of Central Atom-
The charge of the complex if all the ligands are removed along with the electron pairs that are shared with the central atom, is called oxidation number of central atom.e.g., [CU(CN4)3(-ve ions), oxidation number of copper is +1, and represented as Cu(I).