Explain crystal field splitting in a square planar complex.

1. Important terms related to coordination compounds:

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

(ii) 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.

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

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

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

(vi) 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.

(vii) 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.

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

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

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

2. Complexes: 

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

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

(iii) 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.

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

(v) 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.

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

(vii) 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.

(viii) EAN(Effective Atomic No.) $=$ atomic no. $-$ oxidation no. $+(2\times$ coordination no.$)$

3. Theories for existence of coordination compounds:

(i) Crystal Field Theory: 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 $\mathrm{d}$-orbitals are coloured because of intra $\mathrm{d}-\mathrm{d}$ transitions.

(iii) Magnetic moment: It depends upon number of unpaired electrons in orbitals. It is mathematically calculated by expression,

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

(iv) 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: $\mathrm{ }{\mathrm{\beta }}_{\mathrm{n}}={\mathrm{K}}_1\times {\mathrm{K}}_2\times {\mathrm{K}}_3$

(v) The stabilisation of coordination compound due to chelation is called the chelate effect.

4. Bonding in Metal Carbonyls:

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