Chelate ligands: When a di- or polydentate ligand uses its two or more donor atoms to

Coordination Compounds bind a single matal ion it is called chelate ligand.

Werner’s Theory Ambidentate ligands: Ligands which can ligate through two different atoms is called
ambidentate ligand. For example, NO2-, CN-, SCN- etc.
The various points of Werner’s theory are:
Denticity: The number of donor atoms in a polydentate ligand is called its denticity.
(i) In coordination compounds metals show two types of linkages (valences) - primary
(iv) Coordination number: The number of ligand donor atoms to which the metal is
and secondary valencies.
directly bonded is called coordination number. For example, in [Co(NH3)6]3+ and
(ii) The primary valences are normally ionisable and are satisfied by negative ions.
[Co(en)3]3+ the coordination number is 6.
(iii) The secondary valences are non-ionisable. These are satisfied by neutral molecules or
(v) Coordination sphere: The central atom or ion and the ligand attached to it are
negative ions.
enclosed in square bracket and is collectively called as the coordination sphere.
(iv) The ions or groups bound by the secondary linkages to the metal have characteristic
For example in the complex [Co(NH3)6]Cl3, the coordination sphere is
spatial arrangements corresponding to different coordination numbers.
[Co(NH3)6]3+ and Cl- ion is the counter ion.
Difference between double salt and a complex (vi) Coordination polyhedron: The spatial arrangement of the ligand atoms which are
directly attached to the central atom or ion is called coordination polyhedron.
Both double salts as well as complexes are formed by the combination of two or more (vii) Oxidation number: It is defined as the charge carried by the central atom in a
stable compounds in stoichiometric ratio. However they differ in the fact that double salts complex if all the ligands are removed along with the electron pairs that are
dissociates into simple ions when dissolved in water whereas complexes ions do not shared with the central atom. For example in [Co(NH3)6]3+ the oxidation number
dissociate into ions. of Co is +3.
(viii) Homoleptic and heteroleptic complexes:
Some Important Terms Complexes in which a metal is bound to only one kind of donor groups are called
homoleptic complexes. For example, [Co(NH3)6]3+ , [Co(en)3]3+ etc.
(i) Coordination entity: A coordination entity constitutes a central metal atom or ion
Complexes in which a metal is bound to more than one kind of donor groups are
bonded to fixed number of ions or molecules. For example, [Co(NH3)6]3+, [Ni(CO)4]
known as heteroleptic complexes. For example, [Co(NH3)4Cl2]3+.
(ii) Central atom or ion: In a coordination entity the atom or ion to which a fixed number
of ions or groups are bound in a definite geometrical arrangement around it is called
the central atom or ion. For example, in [Co(NH3)6]3+, the central ion is Co3+.
Nomenclature of coordination compounds
(iii) Ligands: The ions or molecules bound to the central atom or ion in the coordination
entity are called Ligands. For example: H2O, NH3, Cl-, H2NCH2CH2NH2 etc.
I. Writing formulas of mononuclear coordination compounds:
Types of ligands: a. The central atom is listed first.
b. The ligands are than listed in alphabetical order.
Unidentate ligands: Ligands which are bound to a metal ion through a single donor atom c. The metal ion and the ligands are enclosed in square bracket.
are called unidentate ligands. For example, Cl- , H2O, NH3 tc. d. The charge of the complex is indicated outside the square bracket as a right
superscript with the number before the sign.
Didentate ligands: Ligands which are bound to a metal ion through two donor atoms are e. The charge of the cation is balanced by the charge of the anion.
called diidentate ligands. For example, H2NCH2CH2NH2 (ethane-1,2-diamine), C2O42-
(oxalate ion)

Polydentate ligands: Ligands which are bound to a metal ion through more than two
donor atoms are called polydentate ligands. For example, ethylenediaminetetraacetate ion
(EDTA4-).
II. Naming of coordination compounds: V. Isomerism in coordination compounds
a. The cation named first in both positively and negatively charged coordination (a) Structural isomers: Structural isomers have different bonds.
entities, (i) Linkage isomers: This type of isomers result from two possible ways of
b. The ligands are named in alphabetical order before the name of the central atom attachment of an ambidentate ligand to the central atom. For example
or ion.
c. Names of the anionic ligands end in –o. [Co(NH3)5ONO]SO4 and [Co(NH3)5NO2]SO4
d. Prefixes mono, di, tri etc., are used ti indicate the n umber of the individual
(ii) Ionsation isomerism: Complexes having same formula but gives different ions in
ligands in the coordination entity.
the solution. For example,
e. Oxidation state of the metal in cation, anion or neutral coordination entity is
indicated by Roman numeral in bracket. [Co(NH3)5Br]SO4 and [Co(NH3)5 SO4] Br
f. If the complex ion is cation, the metal is named same as the element. If the
complex ion is an anion the name of the metal ends with the suffix –ate. (iii) Coordination isomerism: This type of isomerism occurs when both the cations
III. Some common unidentate ligands. and anions are complexes and they differ in the coordination of ligands. For example:

Name Formula Charge Name of ligand [Co(NH3)6][Cr(C2O4)3] and [Cr(NH3)6][Co(C2O4)3]

Ammonia NH3 zero ammine
Water H2O zero aquo or aqua (iv) Solvate isomerism: Solvate isomers differ by whether or not a solvent molecule
Phosphine PH3 zero phosphine is directly linked to the metal ion or it is only present as free solvent molecule in the
Nitrogen oxide NO zero Nitrosyl crystal lattice. For example: [Cr(H2O)6]Cl3 , [Cr(H2O)5Cl]Cl2.H2O and
Carbon monoxide CO zero Carbonyl [Cr(H2O)4Cl2]Cl.2H2O
Thio urea H2NCSNH2 zero thiourea (b) Geometrical isomerism: They differ by the position of different atoms around the
Halide ion X- -1 Halido
central metal atom.
Hydroxide ion OH- -1 hydroxo
Cyanide ion CN- -1 cyano (c) Optical Isomerism: Optical isomers are those which are not Super impossible on their
Carbonate ion CO32- -2 carbonato mirror images.
Sulphate ion SO42- -2 sulphato VI. Valence Bond Theory: According to this theory, the metal atom or ion under the
Nitrite ion NO2- -1 Nitro or nitrito influence of ligands can use its (n-1)d, ns, np orbitals or ns, np, nd orbitals for
Thiocyanate ion SCN- -1 Thiocynato hybridization to yield a set of equivalent orbitals of definite geometry. These
Acetate ion CH3COO- -1 acetato hybridized orbitals are allowed to overlap with ligand orbitals that can donate
Nitrate ion NO3- -1 nitrato electron pairs for bonding.
IV. Some multidentate ligands
Coordination Type of Geometry
Name of the ligand Charge Abbreviation Donor sites No. Hybridisation
Ethane-1,2-diammine zero en 2 4 sp3 Tetrahedral
Oxalate ion, C2O42- -2 ox 2 4 dsp2 Square planar
Dimethylglyoxime ion -1 dmg 2 5 sp3d Trigonal bipyramidal
Ethylenediamine -4 EDTA4- 6 6 sp3d2 Octahedral
6 d2sp3 Ocatahedral
tetraacetate ion

(a) Inner orbital complex: Complexes using inner d orbital (3d) in hybridization are
called inner orbital or low spin or spin paired complex.
(b) Outer orbital complex: complexes using outer d orbital (4d) in hybridization are
called outer orbital or high spin or spin free complex.
VII. Crystal Field Theory: The crystal field theory (CFT) is an electrostatic model
which considers the metal-ligand bond to be ionic arising purely from electrostatic
interactions between the metal ion and the ligand. Ligands are treated as point
charges in case of anions or dipoles in case of neutral molecules. The five d orbitals
in isolated gaseous metal atom or ion have same energy i.e., they are degenerate.
This degeneracy is maintained if a spherically symmetrical field of negative charges
surrounds the metal atom or ion. However when this negative field is due to ligands
in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lifted.
It results in splitting of the d orbitals. The pattern of splitting depends upon the
nature of the crystal field.

Crystal field splitting in Crystal field splitting in

octahedral coordination entities tetrahedral coordination entities

Crystal field splitting energy (∆) :

VIII.
IX. Sdgv
X. dfsda