Valence Bond Theory For Bonding In Coordination Compounds
Valence Bond Theory
This theory is exclusively used to explain the stereochemistry and magnetochemistry of complexes. Some main points of this theory are given below-
1. It concern itself with the oxidation number of the central metal atom in the complex compounds.
2. The electronic configuration of the central metal in complex compound is then written in their oxidation state.
3. The outer orbital of the central metal is reperesented by a box. The electrons of the inner orbitals does not participate in the bonding.
4. The central metal electron in outer orbitals is shown by upward (↑) and downward (↓) arrow.
5. The electrons of the ligan is shown by cross (x).
6. Each ligand donates 2 electrons to the cental metal atom for the formation of M ← L coordinate bond.
7. The metal - ligan bond is formed by the overlapping of orbitals. Greater the overlapping, stronger the bond.
8. A σ bond is formed by the overlapping of a vacant metal orbital and a filled ligand orbital.
9. A π bond is formed by the overlapping of a filled metal orbital and a vacant ligand orbital.
10. The hybridization and geometry of complexes are related to the number of ligands(i.e. coordination number).
sp hybridisation – Linear
sp2 hybridisation – Triangular
sp3 hybridisation – Tetrahedral
dsp2 hybridisation – Square planar
dsp3 hybridisation – Trigonal bypyramidal
d2sp3 hybridisation – Octahedral
d3sp3 hybridisation – Pentagonal bypyramidal
11. Ligands donating an electron pair easily to the central metal atom are called strong ligands (e.g. CN−,CO etc.) and those donating with difficulty are called weak ligands (e.g. halides, water etc.).
12. Under the influence of strong ligands, metal electrons are forced to pair up even contrary to Hund's rule.
13. The magnetic propertis of complexes are governed by the number of unpaired electrons present in electronic configuration of complexes.
μs = n(n+2)1/2 B.M.
where μs is spin only magnetic moment, n is the number of unpaired electrons and B.M.(Bohr Magneton) is unit of magnetic moment.
Complexes having unpaired electrons are paramagnetic while having no unpaired electrons are diamagnetic.
Example
Co(NH3)6]+3
Co is in +3 oxidation state
Structure of this complex is octahedral in which inner 'd' orbital of central metal atom is used due to strong ligand (NH3).
μs = 0 as this complex does not have any unpaired electron.
Merits of Valence bond theory
1. Valence bond theory explains the geometrical shape and magnetic properties of complexes satisfactorily.
2. It explains the formation of inner complexes in the presence of strong ligands and outer complexes in the presence of weak ligands.
3. It explains the back donation of electrons from metal ions to ligands through dπ pπ overlapping.
Limitations of valence bond theory
1. Valence bond theory does not give information regarding magnetic moment due to orbital contribution of electrons.
2. It can not explain the spectral properties of complexes.
3. It does not explain the relative stability of complex compounds.
Inner Orbital Complexes
If the complex is formed by the use of inner d-orbitals for hybridisation (written as dnsp3), it is called inner orbital complex. In the formation of inner orbital complex, the electrons of the metal are forced to pair up and hence the complex will be either diamagnetic or will have lesser number of unpaired electrons. Such a complex is also called low spin complex.
For example, [Fe(CN)6]-3 and [Co(NH3)6]+3 are inner orbital complexes.
[Co(NH3)6]+3-
Outer Orbital Complexes
If the complex is formed by the use of outer d-orbitals for hybridisation (written as sp3dn), it is called an outer orbital complex. The outer orbital complex will have larger number of unpaired electrons since the configuration of the metal ion remains undisturbed. Such a complex is also called high spin complex.
For example, [Fe(H2O)6]+3, [CoF6]- 3 and [Co(NH3)6]+2 are outer orbital complexes.
[Co(NH3)6]+2-
