Coordination Compounds
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].
Double Salts
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 dissociated
into Fe2+, NH+ 4and SO2- 4ions.
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 entity It is the complex ion which carries negative charge.
e.g., [Fe(CN)6]4-
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 (NiCI 2(H2O)4]. Ni is central metal atom. It is
generally transition element or inner-transition element.
3. Ligands
1
, 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- is hexadentate
ligand.
(iv) Ambidentate ligands These are the monodentate ligands which can ligate through two
different sites, e.g., NO-2, SCN-, 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.
2
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