Valence Bond Theory Linkages in Coordination Compounds

What Is The Valence Bond Theory?

Valence bond theory is an old theory based on the bonding of chemicals. Linus Pauling first used it. In the experiment, electrostatic linkage generation between two atoms takes place, through the establishment of an electron density between the nuclei of the two atoms. The valence atomic orbital of one atom either shares spaces or converges with the valence atomic orbital of another atom. This convergence of orbitals enables two antithetical rotated electrons to share a room between the two atoms. This process gives birth to a covalent bond.


Bonding and Properties of Coordination Compounds

A coordination compound is made up of a central metal or ion. It is usually a transformation metal with a guard-wall of neutral particles or anion, known as ligands. The transformation metal is a Lewis acid, and a d-block element in the periodic table made up of valence electrons. The neutral particles are generally Lewis bases with at least one electron pair to contribute to the central atom. 

Ligands may contain neutral like water (H2O), ammonia (NH3), and ethylenediamine (en), or anionic like chloride(Cl-) and cyanide (CN-).  We can classify ligands upon their strength also: they can be robust or feeble. Let’s discuss with a spectra-chemical series which will help to decide the strength of a ligand:

O22- <I- <Br- <S2- <SCN- (S- bonded) <Cl- <N3- <F- <NCO- <OH- <C2O42- <NCS- <CH3CN <py(pyridine) < NH3 <en (ethylenediamine) <bipy (2,2’-bipyridine) <phen (1,10- phenanthroline) <NO2- <pph3 <CN-)


Coordination Compounds Hybridization

Coordination compounds are the metals that can transform. Here the transition metal is the atom lies at the center surrounded by other atoms. Our frequent assumption is that atomic on-orbit of the central atom in a compound blend to create hybrid orbitals. The blending process is known as hybridization. 

The metals which are going through the transition have s, p, and d orbitals. This s, p, and d orbitals are gone through hybridization. 

The valence bond theory explains that because of the ligands' impact, the central atom or ion can use its' (n-1) d, ns, nd orbitals and can intersect with the ligand orbitals of specific shape like an octahedral, tetrahedral, square planer, etc. The hybridized orbitals can intersect with the ligand orbitals that can contribute electron pairs for linkage.

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Here we will discuss with an example of the diamagnetic octahedral complex [Co (NH3)6]3+; cobalt ion has made up with 3d6 electronic structure. The hybridization can be formulated like 


Orbitals of Co+3ion:

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d2sp3 hybridized orbitals of Co3+ can be seen as,

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d2sp3  hybrid
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The mixture does not have any unpaired electron, so it is diamagnetic. All six pairs that come from NH3 molecules obtain the six hybridized orbitals. As the inner d orbital is involved in the hybridization process, the complex, [Co (NH3)6]3+ is called the inner orbitals or low spin or spin-paired complex. The paramagnetic octahedral complex is usually involved in outer orbital (4d) in hybridization (sp3d2). It is called the outer orbital or high spin or spin-free complex.


Valence Bond Theory Postulates

  • The intersection of two almost full valence orbitals of two different atoms results in a covalent bond. The density between two bonded atoms increases because of the intersecting. This provides calmness to the atoms.

  • Exceptionally, if the atomic orbitals get more than one unpaired electrons, more linkages can be formed. Paired electrons cannot participate in such linkage development.

  • A covalent bond often has direction. This type of bond is common to the area of overlapping atomic orbitals.

  • Two types of bonds can be classified as per the design of the intersection. They are pi bond and sigma bond. The covalent bond that is formed by sideways intersecting of atomic orbitals is named as pi bond. A Sigma bond is generated by intersecting an atomic orbital in between the inter nucleus orbits.


Application of Valence Bond Theory

With the theory's application to a coordination compound, the real electrons from the d axis of the transition metal turn into unhybridized d-orbitals. The electrons contributed by the ligand changed into hybridized orbitals with more energy. Then they are filled with electron pairs added by the ligand.

Valence bond theory is involved in stating covalent bond creation in many particles when it passes through the condition of most intersect; it generates the possibility of the creation of the possibly most robust linkages.

FAQ (Frequently Asked Questions)

1. What Are The Drawbacks Of The Valence Bond Theory?

There are certain drawbacks to the Valence Bond Theory. Such as:

  • It fails to provide an accurate description of magnetic data.

  • It can state the color manifested by coordination compounds.

  • It does not come up with the description of the thermodynamic or kinetic stabilities of coordination compounds.

  • The anticipation that the valence bond theory was made according to the tetrahedral and square planar structures of 4- coordinate complexes is not proper.

  • It is not able to differentiate between robust and feeble ligands.

  • It does not provide the tetra-valency of carbon.

  • It does not deal with the energies of the electrons.

  • The predictions it made are all about the localisations of electrons to certain areas.

2. Discuss The Bonding In Ni Complexes?

Generally, robust ligands create the inner axis, diamagnetic complexes, and feeble field ligands create an outer axis, paramagnetic complexes of the compounds. We will now discuss this with formal examples:

We have two coordination compounds, also known as the transition metal complexes, of nickel, NiCl42- and Ni(CN)42-. Chloride and cyanide are the anionic ligands, Ni in the two complexes is in +2 oxidation condition with electron layout Ar 3d8, which will look like 

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In this situation with four incoming ligands, two shapes can be formed; tetrahedral or square planar, as described in the table. Chloride is the feeble content here in context with spectrochemical series and therefore, will create the outer axis. The sp3 hybridization is going to be a tetrahedral NiCl42- complex. Four chloride ligands will form 8 electrons, and the rest metal ions will develop a paramagnetic complex.