Coordination Compounds : Valence Bond theory of Coordination Compounds

Valence Bond theory of coordination compounds

(i)      The suitable number of atomic orbitals of central metal ion (s, p, d) hybridise to provide empty hybrid orbitals.

(ii)     These hybrid orbitals accept lone pair of electrons from the ligands and are directed towards the ligand positions according to the geometry of the complex.

(iii)    When inner d-orbitals i.e. (n-1) d orbitals are used in hybridization, the complex is called – inner orbital or spin or hyperligated complex.

(iv)    A substance which do not contain any unpaired electron is not attracted by 2 magnet. It is said to be diamagnetic. On the other hand, a substance which contains one or more unpaired electrons in the electrons in the d-orbitals, is attracted by a magnetic field [exception O2 and NO]. It is said to be paramagnetic.

Paramagnetism can be calculated by the expression, µs =  \sqrt { n(n+2) }  where μ = magnetic moment.

          s= spin only value and n= number of unpaired electrons.

          Hence, if n = 1, µs =  \sqrt { 1(1+2) } = 1.73 B.M, if n = 3  µs =  \sqrt { 3(3+2) } = 3.87 B.M and so on

On the basis of value of magnetic moment, we can predict the number of unpaired electrons present in the complex. If we know the number of unpaired electrons in the metal complex, then it is possible to predict the geometry of the complex species.

(v)     There are two types of ligands namely strong field and weak field ligands. A strong field ligand is capable of forcing the electrons of the metal atom/ion to pair up (if required). Pairing is done only to the extent which is required to cause the hybridization possible for that Co-ordination number. A weak field ligand is incapable of making the electrons of the metal atom/ ion to pair up.  

          Strong field ligands : CN, CO, en, NH3,H2O, NO, Py.

          Weak field ligands : I, Br, Cl, F, NO3, OH, C2O42–, NH3, H2O

Geometry (shape) and magnetic nature of some of the complexes  (Application of valence bond theory)

(2)     Ligand field theory : According to this theory when the ligands come closer to metal atom or ion, a field is created. This field tends to split the degenerate d-orbitals of the metal atom into different energy levels. The nature and number and number of lignads determine the extent of splitting on the basis of which the magnetic and spectroscopic properties of the complex can be explained.