Valence Bond Theory in Coordination Chemistry

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Valence Bond Theory

• Acc. to VBT complex formation is done is done in 3 steps.
• Control metal ion provide as many as s, p and d orbital which is equal to its coordination no.
• This vacant orbital hybridized though to give equal no. of hybrid orbital which are equivalent    energetically, symmetrically stereo chemically and directed in a particular direction.
• Ligand provide e- pair to each hybrid orbital forming a special type of  covalent bond called as    coordinate covalent bond represented by [] Arrow head point towards M.

After complex formation μ =0

Magnetic moment will decide is this theory is correct not. By magnetic moment instrument μ is found to be equal O.

 Whenever μ decrease with complex formation resulting complex is low spin. Both w.r.t free metals.

Low spin High spin

Spin paired complex spin free complex

Hypo – Ligated  Hyper – Ligated

Decrease μ remain same

Back pairing of electron is thermodynamically unfavorable process. Violation of Hund’s rule    

It requires energy for pairing of electron.

If Ligand are electron donor Si, S, P, C N(Sp³) they are highly polarizable associated with low electro negativity – So, donate electron strongly forming very strong covalent bond and energy  release is able to compensated energy required for pair. But if Ligand is non – polarizable associated with high electro negativity like O, F N (sp² sp) donor. They only from very weak bond X (halogen) 

So, energy released is not sufficient for pairing & they form bond of high energy at outermost d – orbital.

Similar thing is observed in C.N = 4 i.e.

Strong donor Ligand from  square planer complex

Weak donor Ligand from    tetrahedral complex

[Ni (CN) ₄]^-2 

All tetrahedral complexes are always high spin because d orbital are unaffected.

If in C.N = 4 square planer complex is formed than is low spin [Fe L₆] ⁺μ = 5.93 BM

neutral                                                                                   

L →  

Drawback of VBT

Octahedral of complexes of configuration d^0-3 & d^{8 – 10} connect be distinguished as high spin or low spin complexes simply on the basis of magnetic moment.

 Similar case is observed with square planar complexes also.

VBT is able to explain magnetic property of complex but unable to explain colour property of complex.

However these are strong relationship between magnetic & coloured properties.

Generally diamagnetic complexes are colorless e.g. off white, pale colour paramagnetic complexes are generally strong coloured.

VBT is unable to understand the relationship between magnetic & spectral property.

VBT is strongly biased towards coordination no Co & 4 all the octahedral complex of Ni with strong donor Ligand are unstable & change into coordination no 4 losing two Ligands. 

[Ni (NH₃) ₆]⁺³         [Ni (CH₃)₄]²⁺ = 2NH₃

[Ni (CH₃) ₅] is more stable than [Ni (NH₃) ₄] ⁺² but still acc. to VBT 2NH₃ will be loosed.

It is completely failed in explaining structure & hybridization of

 

[Cu (NH₃)₄]⁺²       

By X – ray technique it got cleared that this complex is square planar.

All Co²⁺ with strong Ligand are unstable and oxidies i.e. immediately changes to +3.

VBT explanation [Co (NH₃) ₆] ⁺²    Co²⁺ is highly unstable in ammonia.       

Acc. to VBT single e^- in 4d is highly unstable, so, [Cu (NH₃)₄] ⁺² should be highly unstable acc. to VBT. But it is highly stable.

Most acceptable hybridization is outer orbital but exact stability of [Cu (NH₃)₄] ⁺² is not known to VBT. So, we need another theory that is Crystal field theory (CFT).