Limitations of Werner theory, Valance bond Theory

 

Limitations of Werner theory

 

 Werner was the first to describe the bonding feature in coordination compound but his theory could not explain followings aspects-

·    Certain elements possess the remarkable property of forming coordination compounds

·    The bonds in coordination compound have directional properties.

·    Coordination compounds have characteristic magnetic and optical properties

·    Could not explain the colour and electronic spectra of complex compound

·    Could not explain the pairing of electrons in the presence in the presence of some ligands

·    Could not explain the electrons distribution in the complexes

       many approaches have been forth to explain the nature of bonding in coordination compound. These are as

1). Valence bond theory (VBT)

2). crystal field theory (CFT)

3). ligand field theory (LFT)

4). Molecular orbital theory (MOT)

we will discuss about valence bond theory and Crystal field theory (as per CBSE syllabus)

Valance bond Theory

 Actually it is proposed by L.Pauling and J. L. Slater in 1935.it deals with the electronic structure of central metal ion in its ground state, kind of bonding, geometry and magnetic properties of complexes

This is based on the following assumptions –

·    Metal cation or atom should have empty (n-1) d ns np or ns np nd orbitals for the accommodation of electrons donated by the ligands

·       The Ligands have at least one sigma orbital containing a lone pair of electrons 

·    Empty orbitals of the metal cation or atom are hybridized and the empty hybrid orbitals overlap with the orbital of the ligand and as a result, coordinate or covalent bond is formed.it is represented by M←L.

_·    There are two types of ligand

(a) strong ligand - strong σ-donor example CO, CN^-, NO and

(b) weak ligand- weak σ-donor examples R^- Cl^- Br^- I^- H₂O

Note- generally NH₃ behaves as a weak ligand but in case of Cobalt ion it behaves as strong ligand.

·    Strong ligands have the tendency to pair up the electron in metal orbitals where as weak ligand have no such tendency. As a result of this there are two types of octahedral complex (d²sp³ or sp³d²) are formed

I)     Inner orbital complex- those complexes which is formed by using inner d-orbitals in

hybridization are called inner orbital complexes or low spin complexes.

II)   Outer orbital complex- those complexes which is formed by using outer d-orbitals

are called outer orbital complexes or high spin complexes or spin free complexes.

·   Tetrahedral complexes formed when sp³ hybridisation occurs.

·   Square planner complex also formed when dsp² hybridisation takes place.

·   Complexes having one or more unpaired electrons are called paramagnetic and the

complexes having no unpaired electrons are called diamagnetic

 Magnetic momentum (µ) = √n(n+2)  B.M. (Bohr Magnetons)

here n is no of unpaired electron

NEXT