Notes-Coordination compounds.pdf

UNIT-9
COORDINATION COMPOUNDS
Transition metals form a large number of complex compounds in which the metal atoms are bound to a
number of ions or neutral molecules through coordinate bonds. In modern terminology such compounds
are called coordination compounds.
Inorganic salts:
A salt is a solid chemical compound consisting of an ionic assembly of cations and anions. Salts are
composed of related number of cations and anions so that the product is electrically neutral.
✓ A simple salt is made up of one acid radical and one basic radical. e.g: NaCl, Al2(SO4)3 etc.
✓ A double salt contains more than one cation or more than one anion, and is obtained by the
combination of two different simple salts. e.g:
Carnallite : KCl.MgCl2.6H2O
Mohr’s salt : FeSO4.(NH4)2SO4.6H2O
Potash alum : KAl(SO4)2.12H2O etc.
✓ A complex salt contains at least one complex ion in which a central metal ion is bound to certain
number of ions or molecules through coordinate bonds. e.g:
Potassium ferrocyanide : K4[Fe(CN)6]
Potassium ferricyanide : K3[Fe(CN)6]
Cuprammonium sulphate : [Cu(NH3)4]SO4
Difference between a double salt and a complex salt (coordination compound):
Both double salts as well as complexes are formed by the combination of two or more stable compounds
in stoichiometric ratio. However, they differ with respect to their ionization in water.
When dissolved in water, a double salt completely dissociates into simple ions while a complex salt does
not, i.e. the complex ion remains undissociated.
K4[Fe(CN)6] ⇆ 4 𝐾+
+ [𝐹𝑒(𝐶𝑁)6]4−
➔ totally 5 ions/formula unit
Complex ion
[Cu(NH3)4]SO4 ⇆ [𝐶𝑢(𝑁𝐻3)4]2+
+ 𝑆𝑂4
2−
➔ totally 2 ions/ formula unit
Complex ion
K3[Al(C2O4)3] ⇆ 3 𝐾+
+ [𝐴𝑙(𝐶2𝑂4)3]3−
➔ totally 4 ions/ formula unit
Complex ion
Definitions of Some Important Terms Pertaining to Coordination Compounds
(a) Coordination entity:
A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or
molecules. For example, [CoCl3(NH3)3] is a coordination entity in which the cobalt ion is surrounded by
three ammonia molecules and three chloride ions. Other examples are [Ni(CO)4], [PtCl2(NH3)2],
[Fe(CN)6]4–
, [Co(NH3)6]3+
.
(b) Central atom/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, the central
atom or ion in the coordination entities: [NiCl2(H2O)4], [CoCl(NH3)5]2+ and [Fe(CN)6]3– are Ni2+, Co3+ and
Fe3+
, respectively. These central atoms or ions are also referred to as Lewis acids.

(c) Ligands:
✓ The ions or molecules bound to the central atom or ion in the coordination entity are called ligands.
These may be simple ions such as Cl–
, small molecules such as H2O or NH3, larger molecules such as
H2NCH2CH2NH2 or N(CH2CH2NH2)3 or even macromolecules, such as proteins.
✓ The atom of the ligand which donates electron pairs to the central metal atom or ion is called the
donor atom. Depending on the number of donor atoms present, ligands can be classified as follows.
(i) When a ligand is bound to a central metal atom or ion through a single donor atom, the ligand is said
to be unidentate or monodentate ligand. e.g: Cl–
, CN–
, OH–
, H2O, NH3, CO etc.
(ii) When a ligand can bind through two donor atoms to a central metal atom or ion, the ligand is said to
be didentate or bidentate ligand. e.g: H2NCH2CH2NH2 (ethane-1,2-diamine or en) or C2O4
2–
(oxalate)
etc.
C
H2
C
H2
NH2
NH2
. .
. .
COO
COO
ethane-1,2-diamine / ethylene diamine or en oxalate
(iii)When a ligand can bind through several donor atoms to a central metal atom or ion, the ligand is said
to be polydentate. e.g: NH(CH2CH2NH2)2, N(CH2CH2NH2)3, ethylenediaminetetraacetate ion etc. EDTA4 –
is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a
central metal ion.
C
H2
NH
CH2
C
H2 CH2 NH2
NH2
. .
. .
.
.
Diethylene triamine or dien
(Tridentate ligand)
C
H2
N
CH2
C
H2 CH2 NH2
NH2
CH2
CH2
N
H2
. .
. .
. .
.
.
Triethylene tetraamine or tren
(Tetradentate ligand)
C
H2
C
H2
N
N
CH2COO
CH2COO
CH2COO
CH2COO
. .
. .
Ethylenediaminetetraacetate or EDTA4-
(Hexadentate ligand)
✓ The number of donor atoms (or ligating groups) through which a ligand binds to a central metal
atom/ion in a complex is called the denticity of the ligand. e.g: Denticity of en and EDTA4 –
are 2 and 6
respectively.
✓ When a di- or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it is said
to be a chelate ligand. Such complexes, called chelate complexes.
Chelate complexes tend to be more stable than similar complexes containing unidentate ligands. This
is referred to as chelate effect.
✓ A ligand which can ligate through two different donor atoms but binds to the central metal atom or
ion using any one at a time, is called ambidentate ligand. Examples of such ligands are the CN–
, NO2
–
and SCN–
ions. NO2
–
ion can coordinate either through nitrogen or through oxygen to a central metal
atom/ion. Similarly, SCN– ion can coordinate through the sulphur or nitrogen atom.
CN
M
NC
M
cyanido
isocyanido
SCN
M
NCS
M
thiocyanato
isothiocyanato
N
O
O
M
O N O
M
nitrito-N
nitrito-O
(d) Coordination sphere:
The central atom or ion and the ligands attached to it are enclosed in square bracket and is collectively
termed as the coordination sphere. The ionisable groups are written outside the bracket and are 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+
. Similarly, in the complex [Cu(NH3)4]SO4, the coordination sphere is [𝐶𝑢(𝑁𝐻3)4]2+
and
the counter ion is 𝑆𝑂4
2−
.

(e) Oxidation number of central atom:
The oxidation number of the central atom in a complex is defined as the charge it would carry if all the
ligands are removed along with the electron pairs that are shared with the central atom. The oxidation
number is represented by a Roman numeral in parenthesis following the name of the coordination
entity. For example, oxidation number of copper in [Cu(CN)4]3–
is +1 and it is written as Cu(I).
Complex/Complex ion Oxidation number of central atom (x)
[PtCl6]2–
x + 6 (– 1) = –2
➔x = +4
[Fe(C2O4)3]3–
x + 3 (– 2) = –3
➔ x = +3
[Co(en)3]3+
x + 3 (0) = +3
➔ x = +3
K4[Fe(CN)6] 4 (1) + x + 6 (– 1) = 0
➔x = +2
[Co(NH3)4(H2O)Cl]Cl2 x + 4 (0) + 1 (0) + 1 (–1) + 2(–1) = 0
➔ x = +3
(f) Coordination number:
The coordination number of a metal ion in a complex can be defined as the number of ligand donor
atoms to which the metal is directly bonded.
Numerically, the coordination number of the central atom or ion is equal to the number of sigma
bonds formed by the ligand with the central atom or ion. Hence, it can be determined based on the
denticity of the ligands. Pi bonds, if formed between the ligand and the central atom or ion, are not
counted for this purpose. For example,
Complex/Complex ion Central metal atom/ion Ligand/s Coordination number
[PtCl6]2–
Pt4+
Cl–
: unidentate 1 × 6 = 6
[Ni(NH3)4]2+
Ni2+
NH3 : unidentate 1 × 4 = 4
[Fe(C2O4)3]3–
Fe3+
C2O4
2–
: didentate 2 × 3 = 6
[Co(en)3]3+
Co3+
en : didentate 2 × 3 = 6
K4[Fe(CN)6] Fe2+
CN–
: unidentate 1 × 6 = 6
[Cu(NH3)4]SO4 Cu2+
NH3 : unidentate 1 × 4 = 4
[Co(NH3)2(en)2]Cl3 Co3+
NH3 : unidentate
en : didentate
(1 × 2)+( 2 × 2) = 6
[Ni(CO)4] Ni CO : unidentate 1 × 4 = 4
[Co(NH3)4(H2O)Cl]Cl2 Co3+
NH3 : unidentate
H2O : unidentate
Cl– : unidentate
(1 × 4)+( 1 × 1)+ (1 × 1)= 6
(g) Coordination polyhedron:
The spatial arrangement of the ligand atoms which are directly attached to the central atom or ion
defines a coordination polyhedron about the central atom. The most common coordination polyhedra
are octahedral, square planar and tetrahedral. For example, [Co(NH3)6]3+
is octahedral, [Ni(CO)4] is
tetrahedral and [PtCl4]2–
is square planar. The shapes of different coordination polyhedral are given
below. [M represents the central atom/ion and L, a unidentate ligand]

(h) Homoleptic and heteroleptic complexes:
✓ Complexes in which a metal is bound to only one kind of donor groups, are known as homoleptic
complexes.
e.g: [Fe(CN)6]4–
, [Co(NH3)6]3+
, [Cu(CN)4]3–
, [𝑪𝒖(𝑵𝑯𝟑)𝟒]𝟐+
, [Ni(CO)4] etc.
✓ Complexes in which a metal is bound to more than one kind of donor groups, are known as
heteroleptic complexes.
e.g: [Pt(NH3)2Cl2], [Co(NH3)4Cl2]+
, [Co(NH3)2(en)2]3+
, etc.
Nomenclature of Coordination Compounds:
Formulas of Mononuclear Coordination Entities:
Mononuclear coordination entities contain a single central metal atom. The following rules are applied
while writing the formulas:
(i) The cation is listed first followed by the anion.
(ii) The charge of the cation(s) is balanced by the charge of the anion(s) such that the coordination
compound as a whole is electrically neutral.
(iii) The formula for the entire coordination entity, whether charged or not, is enclosed in square
brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand
abbreviations are also enclosed in parentheses.
(iv) In coordination entity, the central atom is listed first followed by the ligands in alphabetical order.
There should be no space between the ligands and the metal within a coordination sphere.
(v) In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of
the ligand in the alphabetical order. The prefixes like di-, tri-, etc. are not considered for the
alphabetical order.
(vi) When the formula of a charged coordination entity is to be written without the counter ion, the
charge is indicated outside the square brackets as a right superscript with the number before the
sign. For example, [Co(CN)6]3–
, [Cr(H2O)6]3+
, etc.
Naming of Mononuclear Coordination Compounds:
The following rules are used when naming coordination compounds:
(i) The cation is named first in both positively and negatively charged coordination entities followed by
the anion with a space in between.
(ii) The ligands are named in an alphabetical order before the name of the central atom/ion. (This
procedure is reversed from writing formula).
(iii) Names of the anionic ligands end with –o obtained by replacing the last letter –e of their
common names. For example,
Ligand Formula IUPAC name
Halide X–
(e.g: F–
, Cl–
, Br–
, I–
) fluorido /chlorido/bromido/iodido
Cyanide CN–
cyanido
Hydroxide OH–
hydroxido
Thiocyanate SCN–
thiocyanato
Carbonate CO3
2–
carbonato
Oxalate C2O4
2–
oxalato
Nitrite NO2
–
nitrito–O/ nitrito–N
(iv) Names of neutral and cationic ligands are the same except aqua for H2O, ammine for NH3, carbonyl
for CO and nitrosyl for NO.

(v) Prefixes such as di, tri, etc., are used to indicate the number of the individual ligands in the
coordination entity. When the names of the ligands include such prefixes, then the terms, bis, tris,
tetrakis are used and the ligand to which they refer being placed in parentheses.
(vi) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman
numeral in parenthesis.
(vii) In case of neutral complex or complex cation, the metal is named same as the element. For
example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an
anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion, is
called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the
complex anions, e.g., ferrate for Fe, argentate for Ag, aurate for Au etc.
(viii) Number of counter ions is not indicated in the IUPAC name.
(ix) Oxidation number of counter metal ion is indicated only if there are more than one coordination
entities. e.g: Fe4[Fe(CN)6]3
The following examples illustrate the nomenclature for coordination compounds:
Formula IUPAC Name
[Cr(NH3)3(H2O)3]Cl3 Triamminetriaquachromium(III) chloride
[Co(H2NCH2CH2NH2)3]2(SO4)3 Tris(ethane-1,2–diamine)cobalt(III) sulphate
[Ag(NH3)2][Ag(CN)2] Diamminesilver(I) dicyanidoargentate(I)
[NiCl2(PPh3)2] Dichloridobis(triphenylphosphine)nickel(II)
[Pt(NH3)2Cl(NO2)] Diamminechloridonitrito-N-platinum(II)
K3[Cr(C2O4)3] Potassium trioxalatochromate(III)
[CoCl2(en)2]Cl Dichloridobis(ethane-1,2-diamine)cobalt(III) chloride
[Co(NH3)5(CO3)]Cl Pentaamminecarbonatocobalt(III) chloride
Hg[Co(SCN)4] Mercury tetrathiocyanatocobaltate(III)
[Co(NH3)6]Cl3 Hexaamminecobalt(III) chloride
[Co(NH3)5Cl]Cl2 Pentaamminechloridocobalt(III) chloride
K3[Fe(CN)6] Potassium hexacyanidoferrate(III)
K3[Fe(C2O4)3] Potassium trioxalatoferrate(III)
K2[PdCl4] Potassium tetrachloridopalladate(II)
[Pt(NH3)2Cl(NH2CH3)]Cl Diamminechlorido(methanamine)platinum(II) chloride
IUPAC Name Formula
Tetraammineaquachloridocobalt(III) chloride [Co(NH3)4(H2O)Cl]Cl2
Potassium tetrahydroxidozincate(II) K2[Zn(OH)4]
Potassium trioxalatoaluminate(III) K3[Al(C2O4)3]
Dichloridobis(ethane-1,2-diamine)cobalt(III) [CoCl2(en)2]+
Tetracarbonylnickel(0) [Ni(CO)4]
Tetraamminediaquacobalt(III) chloride [Co(NH3)4(H2O)2]Cl3
Potassium tetracyanidonickelate(II) K2[Ni(CN)4]
Tris(ethane–1,2–diamine)chromium(III) chloride [Cr(en)3]Cl3
Amminebromidochloridonitrito-N-platinate(II) [Pt(NH3)BrCl(NO2)]–
Dichloridobis(ethane–1,2–diamine)platinum(IV) nitrate [PtCl2(en)2](NO3)2
Iron(III) hexacyanidoferrate(II) Fe4[Fe(CN)6]3
Werner’s Theory of Coordination Compounds:
• Alfred Werner (1866-1919), a Swiss chemist was the first to formulate his ideas about the structures of
coordination compounds.
• Werner proposed the concept of a primary valence and a secondary valence for a metal ion. Binary
compounds such as CrCl3, CoCl2 or PdCl2 have primary valence (oxidation state of metal) of 3, 2 and 2

respectively. In a series of compounds of cobalt (III) chloride with ammonia, it was found that some of
the chloride ions could be precipitated as AgCl on adding excess silver nitrate solution in cold but some
remained in solution.
1 mol CoCl3.6NH3 (Yellow) gave 3 mol AgCl
1 mol CoCl3.5NH3 (Purple) gave 2 mol AgCl
1 mol CoCl3.4NH3 (Green) gave 1 mol AgCl
1 mol CoCl3.4NH3 (Violet) gave 1 mol AgCl
These observations, together with the results of conductivity measurements in solution can be
explained if
i) six groups in all, either chloride ions or ammonia molecules or both, remain bonded to the
cobalt ion during the reaction and
ii) the compounds are formulated as shown in Table below , where the atoms within the square
brackets form a single entity which does not dissociate under the reaction conditions. The ions
written outside the square bracket dissociate and could be precipitated.
Colour Formula Solution conductivity corresponds to
Yellow [Co(NH3)6]Cl3 1:3 electrolyte
Purple [CoCl(NH3)5]Cl2 1:2 electrolyte
Green [CoCl2(NH3)4]Cl 1:1 electrolyte
Violet [CoCl2(NH3)4]Cl 1:1 electrolyte
Note: The last two compounds in Table have identical empirical formula, CoCl3.4NH3, but distinct
properties. Such compounds are termed as isomers.
Postulates of Werner’s Theory of Coordination Compounds:
The main postulates are:
1. In coordination compounds metals show two types of linkages (valences) – primary and
secondary.
2. The primary valences are normally ionisable and are satisfied by negative ions.
3. The secondary valences are non ionisable and are satisfied by neutral molecules or negative ions.
The secondary valence is equal to the coordination number and is fixed for a metal.
4. The ions or groups bound by the secondary linkages to the metal have characteristic spatial
arrangements corresponding to different coordination numbers.
He further postulated that octahedral, tetrahedral and square planar geometrical shapes are more
common in coordination compounds of transition metals. Thus, [Co(NH3)6]3+
, [CoCl(NH3)5]2+
and
[CoCl2(NH3)4]+
are octahedral entities, while [Ni(CO)4] and [PtCl4]2–
are tetrahedral and square planar,
respectively.
Problem:
On the basis of the following observations made with aqueous solutions, assign secondary valences to
metals in the following compounds.
Solution: Number of moles of AgCl precipitated per mole of the compounds with excess AgNO3
indicates the number of counter ions. The remaining groups/ions remain bonded to the central metal
and indicate the secondary valence/ coordination number. Hence the secondary valences are-
(i) 4 (ii) 6 (iii) 6 (iv) 6 (v) 4

Isomerism in Coordination Compounds:
• Isomers are two or more compounds that have the same chemical formula but a different arrangement
of atoms. Because of the different arrangement of atoms, they differ in one or more physical or
chemical properties. Two principal types of isomerism are known among coordination compounds.
Each of which can be further subdivided.
Isomerism in coordonation compounds
structural isomerism
Stereoisomerism
Linkage ionisation coordination solvate
geometrical optical
(cis-trans AND fac-mer )
• Structural isomers have the same chemical formula but different bonds.
• Stereoisomers have the same chemical formula and chemical bonds but they have different spatial
arrangement of atoms or groups. A detailed account of these isomers are given below.
Structural isomerism:
(i) Linkage isomerism:
It is a type of isomerism in coordination compounds containing ambidentate ligand in which isomers
differ in terms of donor atom of the ambidentate lingand bound to central metal atom or ion. For
example,
[Co(NH3)5(NO2)]Cl2 (yellow) and [Co(NH3)5(ONO)]Cl2 (red)
(ii) Coordination Isomerism:
It is a type of isomerism which arises from the interchange of ligands between cationic and anionic
entities of different metal ions present in a complex. For example,
[Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6]
In [Co(NH3)6][Cr(CN)6], the NH3 ligands are bound to Co3+
and the CN–
ligands to Cr3+
. In its
coordination isomer [Cr(NH3)6][Co(CN)6], the NH3 ligands are bound to Cr3+
and the CN–
ligands to Co3+
.
(iii)Ionisation Isomerism:
✓ It is a type of isomerism which arises when the counter ion in a complex salt is itself a potential
ligand and can displace a ligand which can then become the counter ion. For example,
[Co(NH3)5(SO4)]Br and [Co(NH3)5Br]SO4
✓ The ionisation isomers dissolve in water to yield different ions and thus react differently to various
reagents.
[Co(NH3)5Br]SO4 (aq)+ BaCl2(aq) → BaSO4 (white ppt)
[Co(NH3)5SO4]Br (aq) + BaCl2(aq) → No ppt
[Co(NH3)5Br]SO4 (aq)+ AgNO3(aq) → No ppt
[Co(NH3)5SO4]Br(aq) + AgNO3(aq) → AgBr (pale yellow ppt)
(iv)Solvate Isomerism:
✓ It is a type of isomerism in which isomers differ by whether or not a solvent molecule is directly
bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. For
example,
[Cr(H2O)6]Cl3 (violet) and [Cr(H2O)5Cl]Cl2.H2O (grey-green).
✓ This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent.
✓ This is similar to ionisation isomerism.

Geometric Isomerism:
• This type of isomerism arises in heteroleptic complexes due to different possible geometric
arrangements of the ligands. Important examples of this behaviour are found with coordination
numbers 4 and 6.
• There are two types of geometrical isomerism.
(i) Cis-trans isomerism and (ii) facial-meridional isomerism.
• In a square planar complexes of formula [MX2L2] (X and L are unidentate) and [MABL2] (A,B and L are
unidentate), the two ligands L may be arranged adjacent to each other in a cis isomer, or opposite to
each other in a trans isomer. For example,
Geometrical isomers (cis and trans) of [Pt(NH3)2Cl2]
Pt
Cl
Cl
H3
N
H3
N
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Pt
NH3
Cl
Cl
H3
N
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
cis-[Pt(NH3)2Cl2] trans-[Pt(NH3)2Cl2]
Geometrical isomers (cis and trans) of [Pt(NH3)(H2O)Cl2]
Pt
Cl
Cl
H3
N
H2
O
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Pt
NH3
Cl
Cl
H2O
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
cis-[Pt(NH3)(H2O)Cl2] trans-[Pt(NH3)(H2O)Cl2]
• Other square planar complex of the type [MABXL] (where A, B, X, L are unidentates) shows three
isomers-two cis and one trans. For
Geometrical isomers (two cis and one trans) of [Pt(NH3)BrCl(NO2)] –
Pt
NH3
NO2
Cl
Br
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
_
Pt
NO2
NH3
Cl
Br
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
_
Pt
NH3
Br
Cl
O2
N
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
_
cis- cis- trans-
• Tetrahedral complexes do not show geometrical isomerism because the relative positions of the
unidentate ligands attached to the central metal atom are the same with respect to each other.
• In Octahedral complexes of formula [MX2L4], the two ligands X may be oriented cis or trans to each
other. For example,
Geometrical isomers (cis and trans) of [Co(NH3)4Cl2]+
Co
Cl
NH3
H3N
H3N
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl
NH3
Co
NH3
NH3
H3N
H3N
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl
Cl
+ +
cis-[Co(NH3)4Cl2]+
trans-[Co(NH3)4Cl2]+
This type of isomerism also arises octahedral complexes containing didentate ligands L–L [e.g.,
NH2CH2CH2NH2 (en)] of formula [MX2(L–L)2]. For example,
Geometrical isomers (cis and trans) of [CoCl2(en)2]+

Co
Cl
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl
Co
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl
Cl
+ +
cis-[CoCl2(en)2]+
en
en
en en
trans-[CoCl2(en)2]+
• Another type of geometrical isomerism occurs in octahedral coordination entities of the type [Ma3b3]
like [Co(NH3)3(NO2)3], called facial-meridional isomerism.
If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral
face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron,
we get the meridional (mer) isomer. For example,
The facial (fac) and meridional (mer) isomers of [Co(NH3)3(NO2)3]
Co
NH3
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
NH3
Co
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
NH3
NH3
fac-[Co(NH3)3(NO2)3]
NH3
O2N
O2N
NO2
O2
N
O2N NO2
NH3
mer-[Co(NH3)3(NO2)3]
Optical Isomerism:
• Optical isomers of coordination compounds are mirror images that cannot be superimposed on one
another. These are called as enantiomers. The molecules or ions that cannot be superimposed are
called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate
the plane of polarised light in a polarimeter (d rotates to the right, l to the left).
• Optical isomerism is common in octahedral complexes involving didentate ligands.
• In a coordination entity of the type [PtCl2(en)2]2+
, only the cis-isomer shows optical activity. The trans-
isomer is achiral and optically inactive due to the presence of plane of symmetry.
Optical isomers (d and l) of cis[PtCl2(en)2]2+
:
Pt
Cl
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl 2+
en
en
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
mirror
Pt
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl 2+
Cl
en
en
Optical isomers (d and l) of [Co(en)3]3+
:
Co
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3+
en
en
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
mirror
Co
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3+
en
en
en en
dextro- laevo-
Problem: Draw the structures of two geometrical isomers of [CrCl2(ox)2]3–
. Which isomer is chiral
(optically active)?
Solution: cis-[CrCl2(ox)2]3–
is chiral (optically active) due to the absence of plane of symmetry.

Cr
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3-
ox
ox
Cr
- - - - - - - - - - -
- - - - - - - - - - -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Cl 3-
ox
Cl
Cl
Cl
ox
cis-[CrCl2(ox)2]3–
trans-[CrCl2(ox)2]3–
Limitations of Werner’s theory of coordination compounds:
Werner was the first to describe the bonding features in coordination compounds. But the theory has
following limitations:
(a) It could not explain why only certain elements possess the remarkable property of forming
coordination compounds.
(b) It fails to explain the directional properties of bonds in coordination compounds.
(c) It fails to explain the characteristic magnetic and optical properties exhibited by coordination
compounds.
Many approaches have been put forth to explain the nature of bonding in coordination compounds viz.
Valence Bond Theory (VBT), Crystal Field Theory (CFT), Ligand Field Theory (LFT) and Molecular Orbital
Theory (MOT).
Valence Bond Theory (VBT):
• According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np
or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as
octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with
ligand orbitals that can donate electron pairs for bonding.
• Involvement of inner d-orbitals [(n-1) d orbitals] in hybridisation results in inner orbital complex and
that of outer d-orbitals [nd orbitals] results in outer orbital complex.
• If there is a possibility of two electronic configurations, the one with lesser number of unpaired
electrons is low spin and that with more number of unpaired electrons is high spin complex.
• It is usually possible to predict the hybridisation, geometry and magnetic behaviour of a complex on
the basis of the valence bond theory. This is illustrated by the following examples.
Example 1: [Co(NH3)6]3+
(Atomic No. of Co: 27)
Electronic configuration of Co: [Ar] 3d7
4s2
Electronic configuration of Co3+
: [Ar] 3d6
4s0
3d 4s 4p
Orbitals of Co3+
:
Since NH3 is a strong field ligand, the pairing of electrons takes place in 3d orbitals as follows.
3d 4s 4p
d2
sp3
hybridisation

d2
sp3
hybrid Orbitals of Co3+
:
d2
sp3
hybrid
3d
[Co(NH3
)6
]3+
(inner orbital or
low spin complex) six pairs of electrons from six NH3
ligands
3d
Hybridisation : d2
sp3
Geometry : Octahedral
Magnetic property: Diamagnetic (∵ there are no unpaired electrons)
Example 2: [CoF6]3–
(Atomic No. of Co: 27)
Electronic configuration of Co: [Ar] 3d7
4s2
Electronic configuration of Co3+
: [Ar] 3d6
4s0
3d 4s 4p
Orbitals of Co3+
:
4d
sp3
d2
hybridisation
Since F–
is a weak field ligand, the pairing of electrons does not take place in 3d orbitals.
sp3
d2
hybrid Orbitals of Co3+
:
sp3
d2
hybrid
3d 4d
[CoF6
]3-
(outer orbital or
high spin complex)
six pairs of electrons
from six F-
ligands
3d 4d
Hybridisation : sp3
d2
Geometry : Octahedral
Magnetic property: Paramagnetic (∵ there are unpaired electrons)
Example 3: [NiCl4]2–
(Atomic No of Ni: 28)
Electronic configuration of Ni: [Ar] 3d8
4s2
Electronic configuration of Ni2+
: [Ar] 3d8
4s0
3d 4s 4p
Orbitals of Ni2+
:
sp3
hybridisation
Since Cl– is a weak field ligand, the pairing of electrons does not take place in 3d orbitals.
3d
sp3
hybrid orbitals of Ni2+
:
sp3
hybrid
3d
[NiCl4]2-
(high spin complex)
four pairs of electrons from four Cl-
ligands
Hybridisation : sp3
Geometry : Tetrahedral
Magnetic property: Paramagnetic
Example 4: [Ni(CN)4]2–
(Atomic No of Ni: 28)
4s2
Electronic configuration of Ni2+
: [Ar] 3d8
4s0

3d
Orbitals of Ni2+
:
4s 4p
Since CN–
is a strong field ligand, the pairing of electrons takes place in 3d orbitals as follows.
3d 4s 4p
dsp2
hybridisation
3d
dsp2
hybrid orbitals of Ni2+
:
dsp2
hybrid 4p
3d
[Ni(CN)4]2-
(low spin complex)
4p
four pairs of electrons
from four CN-
ligands
Hybridisation : dsp2
Geometry : Square planar
Magnetic property: Diamagnetic
Example 5: [Ni(CO)4] (Atomic No. of Ni : 28)
In Ni(CO)4, Ni is in zero oxidation state.
4s2
3d
Orbitals of Ni :
4s 4p
Since CO is a strong field ligand, pairing of electrons takes place in 3d orbitals using 4s electrons.
3d
Orbitals of Ni :
4s 4p
sp3
hybridisation
Hybridisation : sp3
Geometry : Tetrahedral
Magnetic property: Diamagnetic
▪ It is possible to predict the geometry of a complex from the knowledge of its magnetic behaviour on
the basis of the valence bond theory. This is illustrated below.
Problem:
The spin only magnetic moment of [MnBr4]2–
is 5.9 BM. Predict the geometry of the complex ion?
Solution: Since the coordination number of Mn2+
ion in the complex ion is 4, it will be either tetrahedral
(sp3 hybridisation) or square planar (dsp2 hybridisation). The magnetic moment of the complex ion is 5.9
BM, indicates the presence of five unpaired electrons in the d orbitals. Hence, dsp2 hybridisation is ruled
out. Thus, complex ion should be tetrahedral in shape.
Limitations of Valence Bond Theory:
(i) It involves a number of assumptions.
(ii) It does not give quantitative interpretation of magnetic data.
(iii) It does not explain the colour exhibited by coordination compounds.
(iv) It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination
compounds.
(v) It does not distinguish between weak and strong ligands
(vi) It does not make exact predictions regarding the tetrahedral and square planar structures of 4-
coordinate complexes.

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 an isolated gaseous metal atom/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/ion. However, when this negative field is due to ligands (either anions or the negative ends
of dipolar molecules like NH3 and H2O) in a complex, it becomes asymmetrical and the degeneracy of
the d orbitals is lost, resulting in splitting of the d orbitals.
• The splitting of the degenerate levels in complexes due to the presence of ligands in a definite
geometry is termed as crystal field splitting. The pattern of splitting depends upon the nature of the
crystal field.
(a) Crystal field splitting in octahedral coordination entities:
• In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be
repulsion between the electrons in metal d orbitals and the electrons of the ligands. The 𝒅𝒙𝟐−𝒚𝟐 and
𝒅𝒛𝟐 orbitals which point towards the direction of the ligand will experience more repulsion and will
be raised in energy; and the 𝒅𝒙𝒚, 𝒅𝒙𝒛 and 𝒅𝒚𝒛 orbitals which are directed away from the ligands will
be lowered in energy relative to the average energy in the spherical crystal field. Thus, splitting of
degenerate orbitals in the octahedral complex yields three orbitals of lower energy, t2g set and two
orbitals of higher energy, eg set.
• The energy separation is denoted by ∆o (the subscript o is for octahedral). The energy of the two eg
orbitals will increase by (3/5) ∆o and that of the three t2g will decrease by (2/5)∆o.
• The crystal field splitting, ∆o, depends upon the field produced by the ligand and charge on the metal
ion. Some ligands are able to produce strong fields in which case, the splitting will be large whereas
others produce weak fields and consequently result in small splitting of d orbitals.
• In general, ligands can be arranged in a series in the order of increasing field strength they produce in
complexes. Such a series is termed as spectrochemical series. It is an experimentally determined
series based on the absorption of light by complexes with different ligands. It is given below:
I–
< Br–
< SCN–
< Cl–
< S2–
< F–
< OH–
< C2O4
2–
< H2O < NCS–
< edta4–
< NH3 < en < CN–
< CO
• For d1
, d2
, d3
, d8
, d9
, d10
system, only one electronic configuration is possible whereas for d4
to d7
system two electronic distributions are possible. For example,
For d4
ions, the two possible patterns of electron distributions are:

(i) the fourth electron could either enter the t2g level and pair with an existing electron, or
(ii) it could avoid paying the price of the pairing energy by occupying the eg level.
Which of these possibilities occurs, depends on the relative magnitude of the crystal field splitting, ∆o
and the pairing energy, P (P represents the energy required for electron pairing in a single orbital).
The two options are:
(i) If ∆o < P, the fourth electron enters one of the eg orbitals giving the configuration t2g
3
eg
1
. Ligands for
which ∆o < P are known as weak field ligands and form high spin complexes.
(ii) If ∆o > P, it becomes more energetically favourable for the fourth electron to occupy a t2g orbital
with configuration t2g
4
eg
0
. Ligands for which ∆o > P are known as strong field ligands and form low
spin complexes.
(b)Crystal field splitting in tetrahedral coordination entities:
• In a tetrahedral coordination entity with four ligands surrounding the metal atom/ion, there will be
repulsion between the electrons in metal d orbitals and the electrons of the ligands. The 𝒅𝒙𝒚, 𝒅𝒙𝒛 and
𝒅𝒚𝒛 orbitals which point towards the direction of the ligand will experience more repulsion and will
be raised in energy; and the 𝒅𝒙𝟐−𝒚𝟐 and 𝒅𝒛𝟐 orbitals which are directed away from the ligands will be
lowered in energy relative to the average energy in the spherical crystal field. Thus, splitting of
degenerate orbitals in the tetrahedral complex yields two orbitals of lower energy, eg set and three
orbitals of higher energy, t2g set.
Note: The ‘g’ subscript is used for the octahedral and square planar complexes which have centre of
symmetry. Since tetrahedral complexes lack symmetry, ‘g’ subscript is not used with energy levels.
• In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as
compared to the octahedral field splitting.
• For the same metal, the same ligands and metal-ligand distances, it can be shown that ∆t = (4/9) ∆0.
Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore,
low spin configurations are rarely observed.
Colour in Coordination Compounds:
• The crystal field theory attributes the colour of the coordination compounds to d-d transition of the
electron.
• The transition metal complexes containing unpaired electrons exhibit wide range of colours. This is
due to the excitation of electron from lower energy to higher energy d-orbital by absorbing light
from visible spectrum. The colour of the complex is complementary to that which is absorbed.
• This means that some of the visible spectrum is being removed from white light as it passes through
the sample, so the light that emerges is no longer white. The complementary colour is the colour
generated from the wavelength left over; if green light is absorbed by the complex, it appears red.

• For example, the complex [Ti(H2O)6]3+
, which is violet in colour. This is an octahedral complex where
the single electron (Ti3+
: 3d1
system) in the metal d orbital is in the t2g level in the ground state of the
complex. The next higher state available for the electron is the empty eg level. If light corresponding to
the energy of blue-green region is absorbed by the complex, it would excite the electron from t2g level
to the eg level (t2g
1
eg
0
→ t2g
0
eg
1
). Consequently, the complex appears violet in colour.
• It is important to note that in the absence of ligand, crystal field splitting does not occur and hence
the substance is colourless. For example, removal of water from [Ti(H2O)6]Cl3 on heating renders it
colourless. Similarly, anhydrous CuSO4 is white, but CuSO4.5H2O {[Cu(H2O)4]SO4.H2O} is blue in colour.
• The influence of the ligand on the colour of a complex may be illustrated by considering the
[Ni(H2O)6]2+
complex, which forms when nickel(II) chloride is dissolved in water. If the didentate ligand,
ethane-1,2-diamine(en) is progressively added in the molar ratios en:Ni, 1:1, 2:1, 3:1, the following
series of reactions and their associated colour changes occur:
[Ni(H2O)6]2+
(aq) + en (aq) ➔ [Ni(H2O)4(en)]2+
(aq) + 2H2O
green pale blue
[Ni(H2O)4 (en)]2+
(aq) + en (aq) ➔ [Ni(H2O)2(en)2]2+
(aq) + 2H2O
blue/purple
[Ni(H2O)2(en)2]2+
(aq) + en (aq) ➔ [Ni(en)3]2+
(aq) + 2H2O
Violet
Limitations of Crystal Field Theory:
a) From the assumptions that the ligands are point charges, it follows that anionic ligands should exert
the greatest splitting effect. But, the anionic ligands actually are found at the low end of the
spectrochemical series.
b) It does not take into account the covalent character of bonding between the ligand & the central atom.
Bonding in Metal Carbonyls:
• The homoleptic complexes containing carbonyl ligands only are formed by most of the transition
metals and are called metal carbonyls.
• These carbonyls have simple, well defined structures.
• Tetracarbonylnickel(0) is tetrahedral, pentacarbonyliron(0) is trigonalbipyramidal while hexacarbonyl
chromium(0) is octahedral.
Decacarbonyldimanganese(0) is made up of two square pyramidal Mn(CO)5 units joined by a Mn – Mn
bond. Octacarbonyldicobalt(0) has a Co – Co bond bridged by two CO groups.
• The metal-carbon bond in metal carbonyls possess both σ and π character. The M–C σ bond is formed
by the donation of lone pair of electrons on the carbonyl carbon into a vacant d-orbital of the metal.
The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the
vacant antibonding π* orbital of carbon monoxide. The metal to ligand bonding creates a synergic
effect which strengthens the bond between CO and the metal.

Importance and Applications of Coordination Compounds:
(A) In analytical chemistry:
✓ Coordination compounds find use in many qualitative and quantitative chemical analysis. The familiar
colour reactions given by metal ions with a number of ligands (especially chelating ligands), as a result
of formation of coordination entities, form the basis for their detection and estimation in analysis.
Examples of such reagents include EDTA, DMG (dimethylglyoxime), α–nitroso–β–naphthol, cupron, etc.
✓ Hardness of water is estimated by simple titration with Na2EDTA. The Ca2+
and Mg2+
ions form stable
complexes with EDTA. The selective estimation of these ions can be done due to difference in the
stability constants of calcium and magnesium complexes.
(B) In metallurgy:
✓ Some important extraction processes of metals, like those of silver and gold, make use of complex
formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the
coordination entity [Au(CN)2]–
in aqueous solution. Gold can be separated in metallic form from this
solution by the addition of zinc.
✓ Similarly, purification of metals can be achieved through formation and subsequent decomposition of
their coordination compounds. For example, impure nickel is converted to [Ni(CO)4], which is
decomposed to yield pure nickel.
(C) In Biological systems:
✓ Coordination compounds are of great importance in biological systems.
▪ The pigment responsible for photosynthesis, chlorophyll, is a coordination compound of magnesium.
▪ Haemoglobin, the red pigment of blood which acts as oxygen carrier is a coordination compound of
iron.
▪ Vitamin B12, cyanocobalamine, the anti– pernicious anaemia factor, is a coordination compound of
cobalt.
▪ Enzymes like, carboxypeptidase A and carbonic anhydrase are coordination compounds of zinc, act as
catalysts of biological systems.
(D) In industrial chemistry:
✓ Coordination compounds are used as catalysts for many industrial processes. Examples include
rhodium complex, [(Ph3P)3RhCl], a Wilkinson catalyst, is used for the hydrogenation of alkenes.
✓ Articles can be electroplated with silver and gold much more smoothly and evenly from solutions of the
complexes, [Ag(CN)2]–
and [Au(CN)2]–
than from a solution of simple metal ions.
✓ In black and white photography, the developed film is fixed by washing with hypo solution which
dissolves the undecomposed AgBr to form a complex ion, [Ag(S2O3)2]3–
.
(E) In medicinal chemistry:
✓ Chelating ligands are used in the treatment of problems caused by the presence of metals in toxic
proportions in plant or animal systems. This is referred to as chelate therapy. Thus,
▪ Excess of copper and iron are removed by the chelating ligands D–penicillamine and desferrioxime
B via the formation of coordination compounds.
▪ EDTA is used in the treatment of lead poisoning.
✓ Some coordination compounds of platinum effectively inhibit the growth of tumours. Examples are:
cis–platin {cis - [Pt(NH3)2Cl2]}and related compounds.
***************************************************************************************

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