Classify the following ligand based on the number of donor atoms.

$\mathrm{en}$

1. Important terms related to coordination compounds:

(i) Swiss chemist Alfred Werner was the first one to propose a theory of coordination compounds to explain the observed behaviour of them.

(ii) Werner’s theory was able to explain a number of properties of coordination compounds, it does not explain their colour and the magnetic properties.

(iii) Double Salts: These addition compounds are stable only in solid state but lose their identity in solution form.

(iv) Co-ordination Compounds: These addition compounds retain their identity in solid state as well as in solution form. In these compounds a central atom is bonded to a number of groups through co-ordinate bonds.

(v) Co-ordination Sphere: The central metal atom/ion along with ligands is written in a square bracket, [ ], called co-ordination sphere.

(vi) Counter ions: The ions present outside the co-ordination sphere/entity are called counter ions.

(vii) Charge on Complex ion: It is algebraic sum of the charges on all the ligands and central atom/ ion.

(viii) Ligands: An atom or group of atoms that can donate a pair of electrons to the central metal atom. These are of the type monodentate, bidentate, tridentate …… polydentate depending upon the number of donor sites.

(ix) Chelating Ligands: Multidentate ligands are chelating ligands if they can be attached to a particular metal atom simultaneously through two or more than two sites.

(x) Ambidentate ligands: Unidentate ligands which have more than one donor atom through which they can co-ordinate to the central atom.

(xi) Co-ordination Number: The total number of ligands attached to the central metal atom in the co-ordination sphere.

(xii) Co-ordination polyhedron: A $3\mathrm{D}$ spatial arrangement of ligands about the central atom.

(xiii) Homoleptic complexes: Complexes in which a metal is bound only by one kind of donor atoms.

(xiv) Heteroleptic complexes: Complexes in which a metal is bound by more than one kind of donor atoms.

2. Nomenclature of coordination compounds:

(i) The cation is named first, followed by the anion regardless of whether the ion is simple or complex.

(ii) To name a complex ion, the ligands are named first followed by the central metal atom/ion. When a complex ion contains more than one kind of ligands they are named in alphabetical order.

3. Isomerism of coordination compounds:

(i) Isomerism is the phenomenon in which more than one coordination compounds having the same molecular formula have different physical and chemical properties due to different arrangement of ligands around the central metal atom.

(ii) Linkage isomers: This type of isomers arises when an ambidentate ligand is bonded to the central metal atom/ion through either of its two different donor atoms.

(iii) Coordination isomers: This type of isomers arises in the coordination compounds having both the cation and anion as complex ions. The interchange of one or more ligands between the cationic and the anionic coordination entities result in different isomer.

(iv) Ionisation isomers: This type of isomers arises when an ionisable counter ion (simple ion) itself can act as a ligand. The exchange of such counter ions with one or more ligands in the coordination entity will result in ionisation isomers.

(v) Geometrical isomerism exists in heteroleptic complexes due to different possible three dimensional spatial arrangements of the ligands around the central metal atom.

(vi) Coordination compounds which possess chirality exhibit optical isomerism similar to organic compounds.

(vii) Solvate isomers : The exchange of free solvent molecules such as water , ammonia, alcohol etc.. in the crystal lattice with a ligand in the coordination entity will give different isomers. These type of isomers are called solvate isomers.

4. Valance bond theory:

(i) Octahedral Complex: A complex in which control atom/cation involves ${\mathrm{sp}}^3\mathrm{ }{\mathrm{d}}^2$ or ${\mathrm{d}}^2\mathrm{ }\mathrm{sp}3$-hybridisation.

(ii) Tetrahedral Complex: A complex in which the central atom/cation involves ${\mathrm{sp}}^3$-hybridisation.

(iii) Square Planar Complex: A complex which has a flat structure and the central atom ion has ${\mathrm{dsp}}^2$ or ${\mathrm{sp}}^2\mathrm{d}-$hybridisation

(iv) Inner Orbital Complex: (Low spin Complex). A complex in which $(\mathrm{n} -1)\mathrm{d}$ orbitals are involved with ns and np-orbitals for hybridisation.

(v) Outer Orbital Complex: When nd-orbitals along with ns-and np-orbitals are involved in hybridisation, outer orbital complex or high spin complex is formed.

5. Effective atomic number rule:

$\mathbf{E}\mathbf{A}\mathbf{N}\mathrm{ }=\mathrm{ }\text{atomic number}-\mathrm{ }\text{oxidation number}+\mathrm{ }\text{(}2\mathrm{ }\times \mathrm{ }\text{coordination number)}$

6. Crystal Field Theory:

(i) The metal ion and ligands are considered as point charges. In the presence of ligand field the degeneracy of d-orbitals is lost and they split into two set of orbitals ${\mathrm{t}}_{2\mathrm{g}}$ and ${\mathrm{e}}_{\mathrm{g}}$.

(ii) Transition metal complexes containing unpaired electrons in d-orbitals are coloured because of intra d-d transitions.

7. Magnetic moment:

It depends upon number of unpaired electrons in orbitals. It is mathematically calculated by expression,

$\mathrm{\mu }=\sqrt{\mathrm{n}(\mathrm{n}+2)} \text{B.M}.$

8. Overall Stability Constant:

$\mathrm{\beta }=\frac{\left[\mathrm{M}{\mathrm{L}}_{\mathrm{n}}\right]}{\left[\mathrm{M}\right]{\left[\mathrm{L}\right]}^{\mathrm{n}}}$

The step wise and overall stability constant $\left({\mathrm{\beta }}_{\mathrm{n}}\right)$ are related as:

${\mathrm{\beta }}_{\mathrm{n}}={\mathrm{K}}_1\times {\mathrm{K}}_2\times {\mathrm{K}}_3$

9. Bonding in Metal Carbonyls:

The metal–carbon bond in metal carbonyls possesses both σ and $\mathrm{\pi }$ character. The ligand to metal is σ bond and metal to ligand is $\mathrm{\pi }$ bond. This unique synergic bonding provides stability to metal carbonyls.