Valence bond theory and its assumptions
In this particular article, Valence bond Theory and its assumptions we are going to discuss the valence bond Theory. And Also about its important assumptions in detail. This theory developed mainly by Pauling.
Assumptions of valence bond theory
This theory is based on the following assumptions:
- The central metal cation or atom makes available a number of vacant s,p and or d-orbitals equal to its coordination number to form the coordinate covalent bond with the orbital on the ligands.
- Since the maximum angular overlap of two orbitals forms the strongest bond. And therefore these vacant atomic orbitals of metal are hybridized to form a new set of equivalent bonding orbital, called hybrid orbitals. These orbitals have the same geometry and same energy. These orbitals also have definite directional properties i.e., these orbitals of the point in the directions of ligands. The geometry and hybridization are related to one another. Once you know the geometry of a complex compound, you automatically know which orbitals of the metal cation or atom uses. The relationship between the geometry of the complex and hybridization is given in the table.
- The bonding in metal complexes arises when a filled ligand orbital containing a lone pair of electrons overlaps a vacant hybrid orbital on the metal cation. Or in other words, atom to form a coordinate covalent bond. ( Figure)
- The magnetic moment and the coordination number of the metal cation or atom decide the hybridization and geometry of the complex. Therefore magnetic moment, coordination number, hybridization, and geometry are related to one another. The knowledge of magnetic moment can be of great help in ascertaining the nature of ligands and type of complex.
Ligands containing lone pair-
Each ligand has at least one orbital containing a lone pair of electrons. Pauling classified the ligands into two categories (1) Strong electron donating ligands or simply strong ligands like CN⁻, CO etc. (2) weak ligands or highly electronegative ligands, like F⁻, Cl⁻, Oxygen containing ligands etc.
5 Strong ligands have the tendency to pair up the d-electrons of metals cation or atom to provide the necessary orbitals for hybridization. But the pairing of electrons does not violet the Hund’s rule of maximum multiplicity. On the other hand, weak ligands do not have the tendency to pair up the d- electrons. I.e., in presence of weak ligands electronic configuration of the d- electrons is the same as in free metal cation or atom.
6 The bond formed between the metal and strong ligands such as CN ⁻, CO is considered to be covalent. On the other hand, the bond formed between the metal and weak or highly electronegative ligands like F⁻ is not covalent but it is ionic.
7 The bond formed between the metal and strong ligands like CN⁻, CO etc is considered to be covalent. This would require in many cases the pairing of d- electrons to provide the necessary orbitals for hybridization.
9 In octahedral complexes, the central metal cation is either d² sp³ or sp³d² hybridized. The d- orbitals Involved in d²sp³ hybridization belong to the inner shell i.e.,( n-1) d- orbitals. And these complexes are called as inner orbital complexes. In case of sp³d² hybridization, the d- orbitals belong to outer most shell i.e., nd- orbitals and the complexes are called outer orbital complexes. The octahedral complexes involving d²sp³ hybridization are most stable than those of sp³d ². The d- orbitals Involved in hybridization in octahedral complexes are dx²-y² and dz².
In tetrahedral complexes, the metal cation or atom is either sp³or sd³ hybridized. The d-orbitals Involved in sd³ – hybridization is dxy,dyz and dzx.
In square planar complexes, the metal cation is dsp²-hybridized. The p- and d- orbitals Involved in dsp² – hybridization are px, py, and dx² – y² leaving pz and pz² orbitals projecting above and below the plane of the complex.
10 In case of second and third series transition metal complexes. The d- orbitals Involved in hybridization are inner orbitals i.e., (n- 1) d- orbitals, i.e., nd orbitals become too diffuse to bond well.
11 The complexes having one or more unpaired electrons are paramagnetic and the complexes having only paired electrons are diamagnetic.