Valence Bond Theory

Important features of Valence Bond Theory (VBT) are :

(1) The central metal ion provides hybrid orbitals as required by the ligands i.e. coordination number to form complex.

(2) each ligand should have at least one lone pair of electron.

(3) The lone pair of electrons of the ligand overlap with empty hybrid orbitals of metal ion.

(4) If d-orbitals are involved in hybridisation that may be either inner viz (n-1) orbitals or the outer viz (n) d orbitals. The complexes formed by these two ways are known as inner orbital and outer orbital complexes respectively. Generally, the inner orbital complxes are low spin and outer orbital complexes are high spin.

(5) If the complex contains one or more unpaired electrons, the complex is paramagnetic and if it does not contain unpaired electron, it is diamagnetic.

Which of the following statement(s) is/are the drawbacks of valence bond theory?

Both Option 1 and Option 2 are correct

The correct answer is option 4. Both Option 1 and Option 2 are correct.

Valence bond theory (VBT) is a useful model for describing chemical bonding in coordination complexes by focusing on the interactions between the metal ion and the ligands. However, it has some limitations:

1. Inability to Determine Thermodynamic Stability: Valence bond theory primarily focuses on the directional nature of bonding and orbital overlap between the metal and ligand orbitals. While it can explain the formation of individual bonds within a complex, it does not provide a comprehensive framework for evaluating the overall stability of a complex based on its energy considerations. Factors such as entropy, enthalpy, and free energy changes, which are crucial for determining thermodynamic stability, are not explicitly addressed by VBT.

2. Limited in Describing Spectral Properties: Valence bond theory does not inherently provide explanations for the electronic transitions and spectral properties observed in coordination complexes. Spectroscopic techniques such as UV-visible absorption spectroscopy, fluorescence spectroscopy, and magnetic susceptibility measurements are commonly used to characterize complexes experimentally. While VBT can provide qualitative insights into bonding, more advanced theories like crystal field theory (CFT) and molecular orbital theory (MOT) are often employed to explain the electronic spectra and magnetic properties of coordination compounds.

Regarding the other options:

Option 3: "Valence bond theory is valid only for a few complexes of Co and Cr": This statement is not entirely accurate. While VBT may be more commonly applied to transition metal complexes, it is not restricted to complexes of only cobalt (Co) and chromium (Cr). Valence bond theory can be applied to a wide range of coordination complexes involving various transition metals and ligands.

In summary, while Valence bond theory provides valuable insights into the bonding interactions in coordination complexes, it has limitations in addressing thermodynamic stability and spectral properties, which are better addressed by other theories such as crystal field theory and molecular orbital theory.