Question

Pick out the complex compound in which the central metal atom obeys EAN rule strictly
A. ${{K}_{4}}[Fe{{(CN)}_{6}}]$
B. ${{K}_{3}}[Fe{{(CN)}_{6}}]$
C. $[Cr{{({{H}_{2}}O)}_{6}}]C{{l}_{3}}$
D. $[Cu{{(N{{H}_{3}})}_{4}}]S{{O}_{4}}$

Answer 1

Hint: EAN (Effective Atomic Number) rule is that in a number of metal complexes the metal atom surrounds itself with sufficient ligands that the resulting effective atomic number is numerically equal to the atomic number of the noble gas element found in the same period in which the metal is situated. This no. represents the total no. of electrons surrounding the nucleus of a metal complex.

Complete Step by Step Answer:
The English chemist Nevil V.Sidgwick made the observation of this EAN rule.By using this rule it is possible to predict the number of ligands in complex compounds and also the products of their reactions.

The EAN rule is often regarded as the “18-electron rule” since if one counts only valence electrons , the total number is 18. A coordination compound is a compound which contains coordinate bonds and these bonds are formed by the donation of electron pairs by some groups called ligands.

The formula to calculate the EAN of any complex compound and to compare it with the nearest noble gas atomic number. We have formula as -$EAN=26-(+2)+2\times 6=26-2+12=36$
$EAN=Z-o.s+2(n)$; where Z=atomic number of the metal atom, o.s= oxidation state of the metal atom,
n=No. of ligands attached to it

Now we need to check all the given options to determine which of them follows the EAN rule strictly –
A. ${{K}_{4}}[Fe{{(CN)}_{6}}]$
- In this complex the atomic number of Iron which is the central metal atom here is 26 and there are 6 ligands attached to it which is cyanide (cyano) ligand and here the oxidation state of the $Fe$ will be calculated as:
$ +4+x+6\times (-1)=0 \\
x-6+4=0 \\
x-2=0 \\
x=2 \\ $
Therefore, the $EAN=26-(+2)+2\times 6=26-2+12=36$; which is the atomic number of noble gas element Krypton only . Thus it obeys the EAN rule strictly.
Since, we get our answer in the first option only so we need not to check the other options and thus, option A will be the correct answer.

Note: The nearest value of the EAN to the respective noble gas element will be regarded as the strictly obeying of this rule as here in other cases also the EAN value is near to the noble gases but in this option it is equal to the noble gas element krypton atomic number and thus it is strictly obeying as it is neither some value greater or smaller than the atomic no. of the noble gas element as that in the other options.