Sidgwick effective atomic number(EAN Rule) for coordination compounds-Effective atomic number calculation and its significance

In this blog post, you will learn about Effective Atomic Number Rule or Sidgwick Effective Atomic Number Theory. You will also learn the formula to calculate Effective Atomic Number. You will also learn about the significance of Effective Atomic Number and limitations of the Sidewick concept .

In 1927, sidewick explained the bonding in complex compound on the basis of electronic theory of valency.

Sidewick, accepted the Lewis concept and introduced a new concept of co-ordinate.

According to sidewick’s/or sidgwick’s concept, the ligand, attached with the central metal ion have atoms, which have at least one unshared electron pairs.

He furthur says that when the ligands are attached with the central metal ion have atoms which have at least one un-shared electron pairs;  they donate the electron pair to the central metal ion. The atom which donated the electron pair to the central metal ion are called donors and which acccpts the electron pair is called acceptor.

According to sidewick, the bond central metal ion is a co-ordinate or dative or semi-polar bond which is not very different from a covalent bond. The bond formed is usually represented by. M<-L

The ligand L denotes a pair of electron to the central metal atom M. thus on the basis of sidewick concept, the structure of complexion [Co(NH3)6]3+

Fig-: formation of 6 co-ordinate bond between 6 ammonia molecules and cobalt ion

In the formation of bonds between cobalt ions and ammonia ligands, the nitrogen atom of each ammonia molecule donates the pair of electron to the central metal cobalt ion and forming 6 co-ordinate covalent bonds.

Thus the central cobalt ion is surrounded by 2 types of electrons. i.e

  1. Electrons of cobalt ion
  2. Electrons of the ligand

Q) What are the limitations of the Sidewick concept?

Ans= The limitations of the Sidewick concept are that when the ligands donate the electron pair to the central metal atom/ion, the metal-ion creates a slightly negative charge which is very unlikely. This makes the complex compound unstable which is not explained by Sidewick, so it is the drawback of the Sidewick model.

Q) Sidewick effective atomic number (EAN Rule)

Sidewick suggested that the when the ligands donate the certain pair of electron to the central metal atom/ion, the total number of electrons on the central metal atom/ion including those gained from ligands in the bonding is called effective atomic number(EAN) of the central metal ion.

This rule can also be stated under 2 statements, they are as follows-:

  1. If the total number of electrons i.e EAN of the central metal atom is equal to the atomic number of the inert gas, such compound are stable and diamagnetic.

2. If the total number of electrons i.e EAN of the central metal atom is not equal to the atomic number of the inert gas, such compounds are unstable and paramagnetic.

Formula to calculate the EAN-:

EAN of the central metal atom=(Z-X)+nY

Where Z=atomic number of the central metal atom

X=oxidation number of the central metal atom

N=No. of ligands

Y=number of electrons donate by each ligand

For eg-: calculate the EAN of the cobalt III in the [Co(NH3)6]3+

Ans-: We know that-:

EAN of Co+3= (Z-X)+nY

=(27-3)+6*2

=24+12

=36

The EAN of the Co+3 ion is equal to the atomic number of Krypton(Kr)(Z=36).

No. of unpaired electrons= At. No of Krypton-EAN

=36-36

=0

So it contains no unpaired electrons. Hence it is stable and diamagnetic.

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