Definition of
important terms pertaining to co-ordination compounds
Coordination
entity is an ion or a neutral molecule, composed of a central atom, usually a
metal and the array of other atoms or groups of atoms (ligands) that are attached
to it. In the formula, the coordination entity is enclosed in square brackets.
For example, in potassium ferrocyanide,
K4[Fe(CN)6], the coordination entity is [Fe(CN)6]4-.
In nickel tetracarbonyl, the coordination entity is [Ni(CO)4].
The
central atom/ion is the one that occupies the central position in a
coordination entity and binds other atoms or groups of atoms (ligands) to
itself, through a coordinate covalent bond. For example, in K 4[Fe(CN)6],
the central metal ion is Fe2+. In the coordination entity [Fe(CN)6]4-,
the Fe2+ accepts an electron pair from each ligand, CN-and
thereby forming six coordinate covalent bonds with them. since, the central
metal ion has an ability to accept electron pairs, it is referred to as a Lewis
acid.
The ligands are the
atoms or groups of atoms bound to the central atom/ion. The atom in a ligand
that is bound directly to the central metal atom is known as a donor atom. For
example, in K4[Fe(CN)6], the ligand is CN-
ion, but the donor atom is carbon and in [Co(NH3)6]Cl3
the ligand is NH3 molecule and the donor atom is nitrogen.
The complex ion of the
coordination compound containing the central metal atom/ion and the ligands
attached to it, is collectively called coordination sphere and are usually
enclosed in square brackets with the net charge. The other ionisable ions, are
written outside the bracket are called counter ions. For example, the
coordination compound K4[Fe(CN)6] contains the complex
ion [Fe(CN)6]4- and is referred as the coordination
sphere. The other associated ion K+ is called the counter ion.
The three dimensional
spacial arrangement of ligand atoms/ions that are directly attached to the
central atom is known as the coordination polyhedron (or polygon). For example,
in K4[Fe(CN)6], the coordination polyhedra is
octrahedral. The coordination polyhedra of [Ni(CO)4] is tetrahedral.
The number of ligand
donor atoms bonded to a central metal ion in a complex is called the
coordination number of the metal. In other words, the coordination number is
equal to the number of σ-bonds between ligands and the central atom.
For example,
i. In K4[Fe(CN)6],
the coordination number of Fe2+ is 6.
ii. In [Ni(en)3]Cl2, the coordination number
of Ni2+ is also 6. Here the ligand 'en' represents
ethane-1,2-diamine (NH2-CH2-CH2-NH2)
and it contains two donor atoms (Nitrogen) Each ligand forms two coordination
bonds with nickel. So,totally there are six coordination bonds between them.
The oxidation state of a
central atom in a coordination entity is defined as the charge it would bear if
all the ligands were removed along with the electron pairs that were shared
with the central atom. In naming a complex, it is represented by a Roman
numeral. For example, in the coordination entity [Fe(CN)6]4-,
the oxidation state of iron is represented as (II). The net charge on the
complex ion is equal to the sum of the oxidation state of the central metal and
the charge the on the ligands attached to it. Using this relation the oxidation
number can be calculated as follows
Net charge = (oxidation
state of the central metal) + [(No. of ligands) X (charge on the ligand)]
Example 1:
In [Fe(CN)6]4-,
let the oxidation number of iron is x :
The net charge: -4 = x +
6 (-1) => x = +2
Example 2:
In [Co(NH3)5Cl]2+,
let the oxidation number of cobalt is x :
The net charge: +2 = x +
5 (0) + 1 (-1) => x = +3
The coordination
compounds can be classified into the following types based on (i) the net
charge of the complex ion, (ii) kinds of ligands present in the coordination
entity.
A coordination compound
in which the complex ion
i. carries a net
positive charge is called a cationic complex. Examples: [Ag(NH3)2]+,
[Co(NH3)6]3+ , [Fe(H2O)6]2+,
etc
ii. carries a net negative
charge is called an anionic complex. Examples: [Ag(CN)2]-,
[Co(CN)6]3- , [Fe(CN)6]4-, etc
iii. bears no net
charge, is called a neutral complex. Examples: [Ni(CO)4], [Fe(CO)5]
, [Co(NH3)3(Cl)3],
A coordination compound
in which
i. the central metal
ion/atom is coordinated to only one kind of ligands is called a homoleptic
complex.
Examples: [Co(NH3)6]3+
, [Fe(H2O)6]2+,
ii. the central metal
ion/atom is coordinated to more than one kind of ligands is called a
heteroleptic complex.
Example, [Co(NH3)5Cl]2+,
[Pt(NH3)2Cl2)]
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