(Chemistry Ch-9) 4.Valence Bond Theory and its Limitations & Magnetic Properties of Coordination Compounds

Valence Bond Theory

  • The metal atom or ion under the influence of ligands can use its (n−1)dnsnp or ns, npnd orbitals for hybridisation, to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar, and so on.

Coordination number

Type of hybridisation

Distribution of hybrid

orbitals in space

4

4

5

6

6

sp3

dsp2

sp3d

sp3d2

d2sp3

Tetrahedral

Square planar

Trigonal bipyramidal

Octahedral

Octahedral

  • These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.

  • Octahedral Complexes

  • The hybridisation involved can be d2sp3 or sp3d2.

  • Inner-orbital or low-spin or spin-paired complexes: Complexes that use inner d-orbitals in hybridisation; for example, [Co(NH3)6]3+. The hybridisation scheme is shown in the following diagram.

  • Outer-orbital or high-spin or spin-free complexes: Complexes that use outer d-orbitals in hybridisation; for example, [CoF6]3−. The hybridisation scheme is shown in the following diagram.

  • Tetrahedral Complexes

  • The hybridisation involved is sp3

  • Example: [NiCl4]2−

  • The hybridisation scheme is shown in the following diagram.

  • Square planar Complexes

  • The hybridisation involved is dsp2

  • Example: [Ni(CN)4]2−

  • The hybridisation scheme is shown in the following diagram.

Magnetic Properties of Coordination Compounds

  • Complexes with unpaired electron(s) in the orbitals are paramagnetic.

  • Complexes with no unpaired electron(s) in the orbitals (i.e., all the electrons are paired) are diamagnetic.

  • Example: [MnCl6]3−, [FeF6]3− and [CoF6]3− are paramagnetic. It can be explained on the basis of the valence bond theory. These coordination compounds are outer-orbital complexes with sp3d2 hybridisation, and are paramagnetic containing four, five and four unpaired electrons respectively. 

On the other hand, [Co(C2O4)3]3is diamagnetic. According to the valence bond theory, it is an inner-orbital complex involving d2sp3hybridisation, with no unpaired electron(s), and is diamagnetic.

Limitations of Valence Bond Theory

  • A number of assumptions are involved.

  • Quantitative interpretation of magnetic data is not given.

  • The exhibition of colour by coordination compounds is not explained.

  • The thermodynamic or kinetic stabilities of coordination compounds are not quantitatively interpreted.

  • Whether a complex of coordination number 4 is tetrahedral or square planar cannot be exactly predicted.

  • Weak and strong ligands cannot be distinguished.

0 comments:

Post a Comment