BONDING IN COMPLEXES
Werner's Co-ordination Theory :
Alfred Werner put forward his concept of secondary valency for advancing a correct explanation for the characteristics of the coordination compounds. The fundamental postulates of Werner's theory are as follows.
(i) Metal possess two types of valencies, namely, primary (principal or ionisable) valency and secondary (auxiliary or non-ionisable) valency.
In modern terminology, the primary valency corresponds to oxidation number and secondary valency to coordination number. According to werner primary valencies are shown by dotted lines and secondary valencies by thick lines.
(ii) Every metal cation in complex compound has a fixed number of secondary valencies for example Pt4 cation has its six secondary valency.
(iii) Primary valency is satisfied by negative ions, whereas secondary valency is satisfied either by negative ions or by neutral molecules.
(iv) Primary valency has non-directional character, where as secondary valency has directional character, there fore a complex ion has its definite geometry eg:- [Co(NH3)6]3 - octahedron.
(v) It is the directional nature of secondary valency due to which co-ordination compound exhibits the phenomenon of isomerism.
Werner's Representation of Complexes
Consider the case of CoCl3.xNH3 where primary valency = 3 and secondary valency = 6.
Various structures are summarised in Table -4.
Secondary valency satisfied by
Primary valency staisfied by
[Co(NH3)6]3+ + 3Cl-
|(B)||CoCl3.5NH3||[Co(NH3)5Cl]Cl2||[Co(NH3)5Cl]2++2Cl-||five (NH3) and one (Cl-)||three (Cl-) including one (Cl-) with dual nature|
four (NH3) and two (Cl-)
three (Cl-) including two (Cl-) with dual nature
three (NH3) and three (Cl-)
three (Cl-) all with dual nature
* From Table 4, It is clear that conduction of the complexes will be in the order D < C < B < A.
* They are represented as
Valence Bond Theory :
It was developed by Pauling. The salient features of the theory are summarised below :
(i) Under the influence of a strong field ligands, the electrons of central metal ion can be forced to pair up against the Hund's rule of maximum multiplicity.
(ii) Under the influence of weak field ligands, electronic configuration of central metal atom and ion remains same.
(iii) If the complex contains unpaired electrons, it is paramagnetic in nature, whereas if it does not contain unpaired electrons, then it is diamagnetic in nature and magnetic moment is calculated by spin only formula.
where n is the number of unpaired electrons in the metal ion.
Table 5 Relation between unpaired electrons and magnetic moment
Magnetic moment (Bohr magnetons)
Number of unpaired electrons
Thus, the knowledge of the magnetic moment can be of great help in ascertaining the type of complex.
(iv) When ligands are arranged in increasing order of their splitting power then an experimentally determind series is obtained named as spectrochemical series.
(v) The central metal ion has a number of empty orbitals for accommodating electrons donated by the ligands.
The number of empty orbitals is equal to the co-ordination number of the metal ion for a particular complex.
(vi) The atomic orbital (s, p or d) of the metal ion hybridise to form hybrid orbitals with definite directional properties. These hybrid orbitals now accept e- pairs from ligands to form coordination bonds.
(vii) The d-orbitals involved in the hybridisation may be either inner (n - 1) d orbitals or outer n d-orbitals. The complexes formed in these two ways are referred to as inner orbital complexes and outer orbital complexes, respectively.
Limitations of valence bond theory
(i) Correct magnetic moment of complex compounds can not be theoretically measured by Valence bond theory.
(ii) The theory does not offer any explanation about the spectra of complex (i.e., why most of the complexes are coloured).
(iii) Theory does not offer any explanation for the existence of inner -orbital and outer -orbital complexes.
(iv) In the formation of [Cu(NH3)4]2+ , one electron is shifted from 3d to 4p orbital. The theory is silent about the energy availability for shifting such an electron.
Such an electron can be easily lost then why does not [Cu(NH3)4]2+ complex show reducing properties.