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 |
Related questions:
(4) What is denticity of NO and NO+ ligands ?
(5) Why all the tetrahedral Complexes are high spin Complexes?
(6)Why Fe(CO)5 is colourless while Fe(bipy)(CO)3 is intensely purple in colour ?
(7) Why [Mn(H2O)6]+2 is colourless although in which Mn+2 ion had five unpaired electrons ?
(8) Why [FeF6]3– is colourless whereas [CoF6]3– is coloured?
(9) Why [Ni(CN)4]-2 is colourless while [Ni(H2O)4]-2 is colour although both have +2 oxidation state and 3d8 configuration ?
Thanks a Lot
ReplyDeleteThank you comments
ReplyDelete