Coordination Compounds Formulas

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INTRODUCTION: The concept of co-ordination compounds arises from the complex formation tendency of transition elements.

Molecular or addition compounds:

AgCN + KCN(simple stable compounds) → KCN. AgCN (Addition or molecular compounds)
K₂SO₄ + Al₂ (SO₄)₃ + 24H₂O → K₂SO₄. Al₂(SO₄)₃.24 H₂O(Alum)

Double salts or lattice compounds:

The addition compounds which are stable in solid-state only but are broken down into individual constituents when dissolved in water are called double salts or lattice compounds. e.g.Camallite (KCl. MgCl₂. 6H₂O)

VARIOUS TERMS USED IN COORDINATION COMPOUNDS

1. Central ion: (Centre of coordination)

2. Ligand: Ligands are normally polar molecules like NH₃ ; H₂O or anions such as Cl^–, OH^–, CN^– etc.

3. Types of Ligands on the basis of number of donor atoms present in them:

(A) Mono or unidentate ligands:
e.g : F^–, Cl^–, Br^–, H₂O, NH₃, CN^–, NO₂^–, ON^–, CO

(B) Bidentate ligands:

Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. Some examples are:
Ethylenediamine (en)

Oxalate (ox)

1, 10-Phenanthroline (o-phen)

Glycinato (Gly)

2,2′ -Dipyridyl (Dipy)

Carbonato

(C) Tridentate ligands:

The ligands having three donor atoms are called tridentate ligands. Examples are :
Diethylene triamine (Dien)

2,2’,2”-Terpyridine (terpy)

(D) Petradentate ligands:

These ligands possess four donor atoms. Examples are:
(Nitriloacetato)

Triethylene tetramine (Trien)

(E) Pentadentate ligands:

They have five donor atoms. For example, ethylenediamine triacetate ion.
Ethylenediamine triacetato ion

(F) Hexadentate ligands:

They have six donor atoms. The most important example is ethylenediamine tetraacetateion.
Ethylenediamine tetraacetato ion (EDTA)

4. Coordination number:

• The number of atoms of the ligands that are directly bond to the central metal atom or ion by coordinate bonds is known as the coordination number of the metal atom or ion.
• It is actually the number of coordinate covalent which the ligands form with the central metal atom or ion

5. Coordination sphere:

6. Oxidation state:

It is a number which represents the electric charge on the central metal atom of a complex ion. The charge of the complex is the sum of the charges of the constituent parts.

7. Effective atomic number (EAN): EAN = atomic number of the metal- number of electrons lost in. ion formation + number of electrons gained from the donor atoms of the ligands

Nomenclature :

• Name of cation is followed by anion
• Name of ligands in alphabetical order with numerical prefixes is followed by name of metal atom (with suffix-ate if complex is anion) and O.S. in Roman number
• For polydentate ligand or ligand the name of which contains a number prefix bis. tris is used.
• Tris written before bridging ligand
• Point of attachment of ambidentate ligand is represented by symbol Ex. [Co(en)₂Cl NO₂] Br
  Ans. chloro bis(ethylenediamine) nitrito-N cobalt (III) Bromide

Name of the ligand:
(a) Anionic ligands ending with ‘ide’ are named by replacing ‘ide’ by suffix ‘O’.

Symbol	Name as ligand
N–	Nitrido
Cl–	Chloro/chlorido
O22-	Peroxo
Br–	Bromo bromido
O2H–	Perhydroxo
CN–	Cyano
S2-	Sulphido
O2-	Oxo
NH2-	Ami do
OH–	Hydroxo

(b) Ligands whose names end in ‘ite’ or ‘ate’ become ‘ito’ or ‘ato’ i.e., by replacing the ending *e’ with ‘o’ as follow s

Symbol	Name as ligand
CO32-	Carbonato
C2O42-	Oxalato
SO42-	Sulphato
NO3–	Nitrato
SO32-	Sulphito
CH3COO–	Acetato
NO3–	Nitrito-N

(c) Neutral ligands are given the same names at the neutral molecules. For example. Ethylene diamine as a ligand is named ethylene diamine in the complex. Flowever some exceptions to this rule are –

HO Aquo NH3 Ammine CO Carbonyl NO Nitrosyl CS Thiocarbonyl

(d) Names of positive ligands ends in ‘ium’ e.g.
NH⁺ (Ammonium)
NO⁺ (Nitrosylium)
NH₂NH₃⁺ (Hydrazinium)

BONDING IN COORDINATION COMPOUNDS

1. Werner’s coordination theory:
(a) Each metal in coordination compound possesses two types of valencies:

• Primary valency or principal valencies or ionisable valencies, satisfied by anions only.
• Secondary valency or nonionisable valencies satisfied only by electron pair donor.

2. Valence Bond Theory:
(a) The number of empty orbitals is equal to the coordination number of the metal ion for the particular complex.

(b) The number of unpaired electrons in the complex, points out the magnetic moment and geometry of the complex as well as hybridisation of central metal ion and vice -versa.
Magetic moment μ = \(\sqrt{n(n+2)}\) where n = no of lone pair

(b) Under the influence of a strong ligand (CN, CO etc ) the electrons can be forced to pair up against the Hund’s rule of maximum multiplicity.
• Hybridisation is d²sp³

(c) Hybridisation is sp³d² in case of weak ligands (X, H₂O etc)

3. Drawback of V.B. Theory

• It describes bonding in coordination compounds only qualitatively
• It does not offer any explanation for the optical absorption spectra of complex
• It does not describe the detailed magnetic properties of coordination compounds.

4. Crystal Field Theory (CFT)

(a) Splitting in Octahedral complex (Δ₀)

(b) Splitting in tetrahedral complex(Δ_r)

(c) splitting in square planer complex

Factors affecting split

• Δ_t = \(\frac{4}{9}\) Δ₀
• 3d < 4d < 5d elements
• m^– < m^2- < m³⁺
• Ligand x < H₂O < NH₃ < CN < CO

Metal carbonyls

• O.S. of metals is always 0
• It has σ & π character
• This bonding is called is synergic or bonding
• Bond length between C & O increase & bond order decrease

ISOMERISM IN COORDINATION COMPOUNDS

Classification

Ionization isomerism:

This is due to exchange of groups or ion between the coordinating sphere and the ionization sphere

Hydrate isomerism:

This type of isomerism is due to presence of different number of water molecules inside and outside the coordination sphere
violet [Cr (H₂O)₆]Cl₃, green [Cr(H₂O)₅Cl]Cl₂.H₂O, dark green [Cr(H₂O)₄Cl₂]Cl.2H₂O

Coordination isomerism:

(a) The type of isomerism occurs when both cation and anion are complex.The isomerism is caused by the interchange of ligands between the two complex ions of the same complex. Examples [Co(NH₃)₆] [Cr(CN)₆] and [Cr(NH₃)₆] [Co(CN)₆]

Linkage isomerism :

(a) This type of isomerism arises due to presence of ambidentate ligands like NO₂^–, CN^– CO and SCN^–. These ligands have two donor atoms but at a time only one atom is directly linked to the central metal atom of the complex.
Example [CO(NH₃)₅NO₂]Cl₂ and [Co(NH₃)₅ ONO]Cl₂

Coordination position isomerism:

This is exhibited by poly nuclear complexes by changing the position of ligands w.r.t. different metal atoms present in the complex
Example

and

Geometrical isomerism:

This isomerism is due to ligands occupying different positions around the central metal atom or ion
Example
Diglycinato platinum (iv) complexes

Stereoisomerism:

> Complex of the type [MA₄B₂], [MA₄BC], [MA₂B₂] [M(AA)₂B₂], [M(AA)₂BC], [M(AA)B₂C₂], [MA₂B₂C₂], [M(AB)₃] show cis-trans isomerism
> [MA₃B₃] show fac-mer isomerism
> cis forms of M(AA) B₂C₂, MA₂B₂C₂, M(AA)₂B₂ and M(AA)₂BC are optically active.
> [M(AA)₃], [M(AB)₃] are also optically active.

Optical isomerism:

A coordination compound which can rotate the plane of polarised light is said to be optically active. This is due tp the absence of elements of symmetry in the complex
Example : Bis – benzoylacetonato – beryllium (II)

8. STABILITY OF COMPLEX

This can be expressed as follows M + L ⇔ ML where
M = metal and L is ligand. The stability constant K for this reaction is as shown.
High value of K indicate stable complex.

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