Valence bond theory (VBT) of coordination complexes

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In this article, we will discuss the covalent bonding present in coordination complexes as per VBT, i.e., valence bond theory. A coordination complex is formed by the coordinate covalent bonding of transition metals with electron-rich species called ligands.

All images in this article are by Ammara W.

The transition metals are present in Groups 3-12 of the Periodic Table. Transition metals exhibit variable valencies as they can use their inner shell electrons in chemical bonding in addition to the electrons present in the outermost shells. The transition metal atoms or ions can act as Lewis acids by accepting electron pairs from the ligands in the coordination complex.

So let’s see how VBT explains the formation of a coordination complex.

What is VBT of coordination complexes- Definition

As per VBT, in a coordination complex, various different atomic orbitals of the central atom hybridize to produce new orbitals of equivalent energy. These are known as hybrid orbitals. The hybrid orbitals accommodate electron pairs provided by the ligand molecules to form coordinate covalent bonds. The number of empty orbitals provided to the ligands determines the coordination number of the central metal atom.

For example, cobalt tetrachloride [CoCl4]2- is a coordination complex formed by the chemical bonding of the central Co2+ ion with 4 Cl ions, one on each side. In this case, cobalt is the central metal, while Cl ions act as ligands, containing lone pairs of electrons.

The electronic configuration of the Co-atom is [Ar] 3d7 4s2.

Co2+ is formed by the loss of 2 valence electrons. So the electronic configuration of Co2+ is 1s2 2s2 2p6 3s2 3p6 3d7 4s0 4p0.

During [CoCl4]2- formation, as per VBT, one s and three p atomic orbitals of Co2+ hybridize to yield four equivalent sp3 hybrid orbitals.  The electron pair from each Cl-atom is accommodated in four empty sp3 hybrid orbitals of central Co2+ to form [CoCl4]2-. The complex adopts a tetrahedral shape as per VSEPR theory.  

Different types of hybridization present in coordination complexes

As per the valence bond theory, in coordination complexes, the central metal atom or ion can be:

  • sp hybridized
  • sp2 hybridized
  • sp3 hybridized
  • dsp2 hybridized
  • dsp3 hybridized
  • d2sp3 hybridized
  • sp3d2 hybridized

The coordination complex adopts a linear shape or geometry if the central atom is sp hybridized in it. For example, in [Ag(NH3)2]+, the central Ag+ ion is sp hybridized. It is bonded to two NH3 ligands, one on each side. The ammonia molecule donates an electron pair which is accommodated in an sp hybrid orbital of silver.

The central Hg2+ ion is sp2 hybridized in [HgI3], and it possesses a trigonal planar shape.

In addition to [CoCl4]2-, other examples of tetrahedral coordination complexes are [ZnCl4]2-, [NiCl4]2-, [Ni(CO4)] etc. In each of these examples, the central metal ion is sp3 hybridized.

However, the hybridization present in [Ni(CN)4]2- is dsp2, and it has a square planar shape despite the fact that four ligands are attached to the central Ni2+ in this case as well. VBT explains this difference.

The electronic configuration of Ni2+ is 1s2 2s2 2p6 3s2 3p6 3d8 4s0 4p0.

While [Ni(CN)4]2- formation, the unpaired 3d electrons of nickel get paired up. This results in an empty 3d atomic orbital. One d orbital of the (n-1) shell, one s and two p atomic orbitals of the nth shell (where n= principal quantum number of outermost shell) combine to form four dsp2 hybrid orbitals. Each dsp2 hybrid orbital of central Ni2+ accommodates an electron pair from CN to form a square planar coordination compound.

In the above complex, as the d-orbital of the inner shell is involved in chemical bonding thus, it is known as an inner orbital coordination complex.

Additionally, there are no unpaired electrons in this complex, thus it is a low spin or spin-paired complex, exhibiting diamagnetic behavior.

[Fe(CO)5] is an example of a trigonal bipyramidal complex in which the central Fe-atom is dsp3 hybridized.

How does VBT explain the formation of inner and outer orbital complexes

A coordination complex in which the central metal atom or ion is d2sp3 hybridized is known as an inner orbital complex.

In contrast, the coordination complex formed by sp3d2 hybridization of the central atom or ion is known as an outer orbital complex.

Let’s understand this concept more clearly with the help of examples.

[Cr(NH3)6]3+ is an inner orbital complex.

The electronic configuration of Cr3+ is 1s2 2s2 2p6 3s2 3p6 3d3 4s0 4p0.

 During chemical bonding, two 3d atomic orbitals, one 4s and three 4p orbitals hybridize to yield a total of six d2sp3 hybrid orbitals. As the hybrid orbitals are formed using d orbitals of the inner shell (penultimate shell) thus, it is known as an inner orbital complex. Each d2sp3 hybrid orbital readily accepts an electron pair from NH3 to form [Cr(NH3)6]3+. There are unpaired electrons in 3d orbitals of Cr3+; thus, it is a high-spin complex, paramagnetic in nature.

[Co(NH3)6]3+, on the other hand, is an outer orbital complex.

The electronic configuration of Co3+ is 1s2 2s2 2p6 3s2 3p6 3d6 4s0 4p0 4d0.

In this case, one 4s and three 4p atomic orbitals of Co3+ hybridize with two 4d orbitals to produce six sp3d2 hybrid orbitals. As outer shell d orbitals are involved in this hybridization thus, the coordination complex formed in this case is an outer orbital complex.

Both the above coordination complexes have an octahedral shape. To a metal atom at the center, six ligands are covalently bonded.

An inner orbital complex is usually formed when strong field ligands such as CO and CN approach a metal atom or ion. These can force the pairing of d-electrons.

Contrarily, outer orbital complexes are formed in the presence of weak field ligands such as H2O, Cl, Br, and F, unable to pair up inner shell d-electrons.

You can have a look at different strength ligands through the spectrochemical series. The valence bond theory is quite valuable in predicting the hybridization, shape and geometries of coordination complexes. However, it still lags in explaining other properties, such as the color of coordination complexes which you will find through the crystal field theory (CFT).

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