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SIDWICK THEORY AND EAN RULE:

According to Sidwick Central metal ion of coordination complex will continue accepting electron pairs till the total number of electrons in the metal ion and those donated by ligands is equal to that of the next higher noble gas. This total number of electrons is called effective atomic number (EAN) of the metal. However this rule is more valid for non classical complexes specially carbonyl compounds hence this rule used for determining Stoichiometry, stability. Oxidizing and reducing character of carbonyl compounds.
CALCULATION OF EAN:
Total number of electrons possessed by the central atom in a complex is called Effective Atomic Number (EAN).
EAN = Z-(OS) Oxidation State + 2*coordination Number
This is Not applicable for pi complex
EAN = Z-(OS) Oxidation State + Total Number of electrons gained from ligands

For fast calculations remember the atomic number of the noble gases which are given as

He
2

Ne
10

Ar
18

Kr
36
for 3d series metals
Xe
54
for 4d series metals
Rn
86
for 5d series metals

ILLUSTRATIVE EXAMPLES OF E.A.N. CALCULATION:

SN
COMPLEX
EAN CALCULATION
EAN
1
[Co(CO)4]-
27+1+ 4*2
36
2
[Fe(Co)(NO)3]+
26-1+2+3*3
36
3
[Mo(Co)(NO)3]
25-0+2+3*3
36
4
[Cr(Co)6]
24-0+2*6
36
5
[Fe(pi-(C5H5)2]0
26-2+2*6
36
6
[Cr(pi-(C6H6)2]0
24-0+2*6
36
6
[Ru(CO)5]
44-0+2*6
54
7
[V(CO)6]
23-0+2*6
35
8
[Ti(Sigma-C2H5)(pi-C5H5)]
22-4+2*2+2*6
34
9
[PtCl3(pi-C2H4)]
78-2+2*3+2
84
10
[Co(NH3)6]Cl3
27-3+2*6
36

IMPORTANT NOTE:
(1) The EAN rule is generally found to be not valid in case of most of the complexes but in case of metal carbonyls (which is an important class of complexes, we will be studying later) this rule is found to be valid in all cases except one or two exceptions, so do remember it for metal carbonyls and do know how to calculate the EAN for any metal.
(2) NO ligand is found to act as three electron donor, as indicated by the following reactions in which when the carbonyl compound is heated in atmosphere of excess of NO. But also remember that NO+ is only two electron donor

[Fe(CO)5] + 2NO à [Fe(CO)2(NO)2] + 3CO

[Cr(CO)6] + 4 NO à[Cr(NO)4] + 6 CO


EAN FOR POLYNUCLEAR CARBONYL:  
Only CO is the ligand which can act as bridge ligand which donates only one electron to the one central atom.
[Fe(CO)9] , [Fe3(CO)12] , [Co2(CO)8] , [Mn2(CO)10] , [Ru3(CO)12] , [Re2(CO)10]

Direct Shortcut method:
ILLUSTRATIVE EXAMPLE (1): Calculate EAN of Fe2(CO)9 and Ru3 (CO)12 ?
SOLUTION:
(A) Calculate of EAN of Fe2(CO)9  !

(B) Calculate of EAN of Ru3 (CO)12  !
USES OF EAN RULE: However this EAN rule is more valid for non classical complexes specially carbonyl compounds hence this rule used for determining Stoichiometry, stability. Oxidizing and reducing character of carbonyl compounds.

In general if a complex have EAN equal to the Atomic Number of one of the inert gas then complex is stable however exception is also there for example  EAN of [Fe(CN)6]4- is 36 while EAN of [Fe(CN)6]3- is 35 But [Fe(CN)6]3- is more stable than  [Fe(CN)6]4- due to more effective charge in Fe+3 .
1
[Fe(CN)6]4-
26-2+2*6
36
2
[Fe(CN)6]3-
26-3+2*6
35


ILLUSTRATIVE EXAMPLE (2): Find the  value of (x) in the following compounds (1) [Co2(CO)x]  , (2) Hx[Mn(CO)5] , (3) [Fe(CO)(NO)x]+ , (4) Cr(CO)x


SOLUTION: Such type of question we will always consider that EAN is valid,so  with help of EAN we will find x

SN
COMPLEX
EAN CALCULATION
EAN
Value of x
1
[Co2(CO)x]
27*2 +1*2+ 2*x
36
X=8
2
[Fe(CO)(NO)x]+
26-1+2+ 3*x
36
X=3
3
[H x Mn(CO)5]
25+x+2*5
36
X=1
4
[Cr(Co)x]
24-0+2*x
36
X=6

ILLUSTRATIVE EXAMPLE (2):  Assign the oxidizing and reducing character to the following compounds?  [V(CO)6] , [Fe(CO)2(NO)2] , [Mn(CO)5]+ and [Co(CO)4]2-

SOLUTION:

SN
COMPLEX
EAN
Nature of compounds
1
[V(CO)6]
35
Reducing agent
2
[Fe(CO)2(NO)2]
36
Oxidizing agent
3
[Mn(CO)5]+
34
Oxidizing agent
4
[Co(Co)4]2-
37
Reducing agent

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