Valence Bond Theory of Coordination Compounds - Practice Questions & MCQ

According to this theory, the metal atom or ion under the influence of ligands can use its (n-1)d, ns, np or ns, np, nd orbitals for hybridisation to yield a set of equivalent orbitals of definite geometry such as octahedral, tetrahedral, square planar and so on. These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding. 

For example, [Ni(CN)₄]^2–, the hybridisation of this comples is dsp². Here nickel is in +2 oxidation state and has the electronic configuration 3d⁸.

Each of the hybridised orbitals receives a pair of electrons from a cyanide ion. The compound is diamagnetic as evident from the absence of unpaired electron.

[Cu(NH₃)₄]²⁺ is an exception for determining the hybridisation. On the experimental basis, it has been found that its geometry is square planar, thus atleast one d orbital is compulsory.

The electronic configuration of [Cu(NH₃)₄]²⁺ is as follows:

But in this case, the last electron can easily be removed and Cu²⁺ will become Cu³⁺, which does not exist in reality. Thus to explain its square planar geometry, Huggins proposed sp²d hybridisation that this last unpaired electron remains in the 3d orbital and the hybridisation is done by 4s orbital, 2 4p orbitals and 1 4d orbital. The third 4p orbital does not participate in hybridisation. Thus, in this way, its hybridisation is dsp² and the geometry is square planar.

The magnetic moment of coordination compounds can be measured by the magnetic susceptibility experiments. The results can be used to obtain information about the number of unpaired electrons and hence structures adopted by metal complexes.
A critical study of the magnetic data of coordination compounds of metals of the first transition series reveals some complications. For metal ions with upto three electrons in the d orbitals, like Ti³⁺ (d¹); V³⁺(d²); Cr³⁺(d³); two vacant d orbitals are available for octahedral hybridisation with 4s and 4p orbitals. The magnetic behaviour of these free ions and their coordination entities is similar. When more than three 3d electrons are present, the required pair of 3d orbitals for octahedral hybridisation is not directly available (as a consequence of Hund’s rule). Thus, for d⁴(Cr²⁺, Mn³⁺), d⁵(Mn²⁺, Fe³⁺), d⁶(Fe²⁺, Co³⁺) cases, a vacant pair of d orbitals results only by pairing of 3d electrons which leaves two, one and zero unpaired electrons, respectively.
The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing d⁶ ions. However, with species containing d⁴ and d⁵ ions there are complications. [Mn(CN)₆]^3– has magnetic moment of two unpaired electrons while [MnCl₆]^3– has a paramagnetic moment of four unpaired electrons. [Fe(CN)₆]^3– has magnetic moment of a single unpaired electron while [FeF₆]^3– has a paramagnetic moment of five unpaired electrons. [CoF₆]^3– is paramagnetic with four unpaired electrons while [Co(C₂O₄)₃]^3– is diamagnetic. This apparent anomaly is explained by valence bond theory in terms of formation of inner orbital and outer orbital coordination entities. [Mn(CN)₆]^3–, [Fe(CN)₆]^3– and [Co(C₂O₄)₃]^3– are inner orbital complexes involving d2sp3 hybridisation, the former two complexes are paramagnetic and the latter diamagnetic. On the other hand, [MnCl₆]^3–, [FeF₆]^3– and [CoF₆]^3– are outer orbital complexes involving sp³d² hybridisation and are paramagnetic corresponding to four, five and four unpaired electrons.

While the VB theory, to a larger extent, explains the formation, structures and magnetic behaviour of coordination compounds, it suffers from the following shortcomings:
(i) It involves a number of assumptions.
(ii) It does not give quantitative interpretation of magnetic data.
(iii) It does not explain the colour exhibited by coordination compounds.
(iv) It does not give a quantitative interpretation of the thermodynamic
or kinetic stabilities of coordination compounds.
(v) It does not make exact predictions regarding the tetrahedral and
square planar structures of 4-coordinate complexes.
(vi) It does not distinguish between weak and strong ligands.