Effective atomic number Rule

 

1.What is effective atomic number rule(EAN)?

2.Comparison of effective atomic number rule and 18 electron rule.

3.Limitations of EAN Rule.

Effective atomic number

Effective atomic number rule was put for forward by Sidgwick and Bailey. It is observed by transition metal carbonyls which shows diamagnetic behavior. The composition of stable binary metal carbonyls is largely predictable by EAN rule.

The effective atomic number rule predicts that metal in zero or low oxidation state will gain electrons from sufficient number of ligands so that metal will achieve the electronic configuration of next noble gas.

Counting Rules

1.  Count two electrons for each CO.

2. Count one electron for each metal-metal bond.

3. Find the number of electrons that formally belong to the metal atom alone by

(a) adding up the charges on the ligands and changing the sign ,(b) finding the metal oxidation number by adding this number to the total charge on the complex, and (c) subtracting the oxidation number from the valence electrons count of the neutral metal.

4. Add together the counts from step 1-3.

Examples:

In [Zn(NH3)4]²⁺,  Zn²⁺ has 28 electrons, and receiving 8 more electrons (two from each of four ligands) results in 36 electrons around Zn2+.

 

 

Structure of zinc tetra amine

In [Co(CN6)]^3_, Co³⁺ has 24 electrons and six pairs of electrons from six ligands will raise the number to 36.

 

Structure of cobalt hexacyanide

 

In Fe2(CO)9, let us calculate EAN:

No if electron at Fe= 26

No of electrons donated by 3 terminal CO= 6

No of electrons donated by 3 bridging CO= 3

No of electrons donated by 1 Fe-Fe bond= 1

Thus total number of electrons around each Fe central atom are 36.

Comparison Between EAN and 18-Electron Rule

The Effective Atomic Number (EAN) rule and the 18-electron rule both predict stability of transition metal complexes, but the EAN rule calculates the total electron count (metal + ligands ± charge) to match a noble gas configuration, while the 18-electron rule is a simplified guideline focusing only on valence electrons filling the s, p, and d orbitals.

       Effective Atomic Number (EAN) Rule

Definition: Proposed by Nevil Sidgwick, the EAN rule states that a stable metal complex tends to have a total electron count (metal’s atomic number + electrons donated by ligands ± charge) equal to the nearest noble gas configuration.

Focus: It considers both core electrons and valence electrons, aiming for a full noble gas shell.

Application: Useful in classical coordination chemistry for predicting stability and coordination numbers.

Example: In $[Ni(CO{)}_4]$, Ni has atomic number 28. Adding 8 electrons from 4 CO ligands gives 36, which matches krypton’s configuration → stable complex.

18-Electron Rule

Definition: States that stable transition metal complexes often have 18 valence electrons, filling the s, p, and d orbitals completely.

Focus: Only considers valence electrons (metal + ligand contributions), not the entire atomic number.

Application: Widely used in organometallic chemistry, especially for carbonyls and metal complexes with π-acceptor ligands.

Example: In $[Cr(CO{)}_6]$, Cr contributes 6 valence electrons, and 6 CO ligands donate 12 → total 18 valence electrons → stable.

Limitation: Exceptions exist (e.g., stable 16-electron complexes like $[PtC{l}_2(PP{h}_3{)}_2]$) due to ligand strength, steric effects, or special bonding.

 

Limitations of EAN Rule:

1.     EAN assumes noble gas stability is the only criterion, but many stable                               complexes deviate from this.

2.     It doesn’t explain reactivity, color, or geometry — only electron count.

3.     Modern theories (Crystal Field Theory, Ligand Field Theory, and Molecular Orbital Theory) provide more accurate predictions.

4.     This rule is not followed by 16 electron compounds, including both high spin octahedral complexes and low spin square planar.

5.      This rule does not employ if organometallic compound contains 6 or more atoms.