Reactions of complexes

reactions of metal complexes

1. V.SANTHANAM
Department of Chemistry
SCSVMV

2. Complexes in which exchange of one or
more ligands are rapidly exchanged are
called labile complexes.
If the rate of ligand exchange is slow then
the complex is said to be inert.
Lability is not related to the thermodynamic
stability of a complex.
A stable complex may be labile or inert , so
as the unstable complex .

3. [Cu(NH
3
)
4
(H
2
O
2
)
2
]
2+
is labile. Its aqueous
solution is blue in color.
When concentrated hydrochloric acid is
added to this solution, the blue solution
immediately turns green ,giving [CuCl
4]

2-
.
But when the complex is kept as such it
remains as such with out any decomposition
(i.e stable)

4. [Co(NH
3
)
6
]
3+
reacts slowly. When this complex
is treated with concentrated HCl, no reaction
takes place. Only when it is heated with 6M
HCl for many hours, one NH
3
is substituted by
Cl
-
.
[Co(NH
3)
6]
3+
+ HCl [Co(NH
3)
5Cl]
2+
+ NH
4
+

5. Size of the central metal ion
Smaller the size of the metal ion, greater will be the
inertness because the ligands are held tightly by
the metal ion.
Charge on the central metal ion
Greater the charge on the metal ion, greater will be
the inertness of the complex. Since the M-L bonds
are stronger.

6. d-electron configuration
If electrons are present in the antibonding e
g
*
orbitals,
the complex will be labile -the ligands will be weakly
bonded to the metal and hence can be substituted
easily.
Complexes with empty t
2g orbitals, will be labile
because ligands can approach easily without much
repulsion.
In short, if the complex contains less than three d-
electrons, it will be labile. Or, if one or more e
g
*

electrons are present, it will be labile

7. No. of d electrons
& electron configuration
Nature Example
d
0
Labile [CaEDTA]
2-
d
1
; t
2g
1
e
g
0
Labile [Ti(H
2
O)
6
]
3+
d
2
; t
2g
2
e
g
0
Labile [V(phen)
3
]
3+
d
3
; t
2g
3
e
g
0
Inert[V(H
2
O)
6
]

3+
d
4
(high-spin); t
2g
3
e
g
1
Labile [Cr(H
2
O)
6
]
3+
d
4
(low-spin); t
2g
4
e
g
0
Inert[Cr(CN)
6]
4-
d
5
(high-spin); t
2g
3
e
g
2
Labile [Mn(H
2
O)
6
]
2+
d
5
(low-spin); t
2g
5
e
g
0
Inert[Mn(CN)
6
]
4-
d
6
(high-spin); t
2g
4
e
g
2
Inert[Mn(H
2
O)
6
]
2+
d
6
(low-spin); t
2g
6
e
g
0
Inert[Fe(CN)
6
]
4-
d
7
, d
8
, d
9
, d
10
Labile

8. CFT assumes the splitting of d orbitals of metal.
Filling of e
-
s in them results in different CFSE.
CFAE – Crystal Field Activation Energy
CFAE = CFSE of intermediate – CFSE of Reactant
Since the geometries of the reactant and intermediate
are different their splitting and CFSE are also different.

9. If the calculated CFAE is negative or zero or low
the reacting complex will require less energy to
form the intermediate, hence it will be labile.
If CFAE is a high positive value then the complex
will be inert.
It must be borne in mind that CFAE is only a part
of actual AE and other factors are also operative.

10. The geometry of the complex is assumed to
be O
h
even if all the ligands are not identical.
The inter electronic repulsions are neglected.
The Dq values of the reactant and
intermediate are assumed to be same.
The Jahn-Teller effect is not affecting CFSE.

11. Because of the drastic assumptions made,
some of the CFAE values are –ive.
However when calculated with proper
attention to all effects, CFAE is always +ive.
CFAE can be small or zero but never –ive
By oversimplified approach the –ive values of
CFAE may be taken as zero.

12. Substitution of ligands
Solvolysis
Anation
Reactions of coordinated ligands
Racemization
Electron transfer reactions
Photo chemical reactions

13. • Ligand displacements are nucleophilic
substitution reactions.
• Rate is governed by ligand
nucleophilicity
The rate of attack on a complex by a
given ligand relative to the rate of attack
by a reference base.
• Rates span from 1 ms to 10
8
s

14. Three types of ligands are present
– Entering Ligand: Y
– Leaving Ligand: X
– Spectator Ligand
• Species that neither enters nor leaves
• Particularly important when located in a Trans
position, designated T

16. Dissociative: One of the ligands
dissociates from the reactant, to form a
reaction intermediate with lower
coordination number than reactants
or products
• Octahedral complexes and smaller metal centers
• Rates depend on leaving group

21. SYSTE
M
Weak Field / High Spin Strong Field / Low Spin
O
h
SP CFAE O
h
SP CFAE
d
0
0 0 0 0 0 0
d
1
-4 -4.57 -0.57 -4 -4.57 -0.57
d
2
-8 -9.14 -1.14 -8 -9.17-4-1.14
d
3
-12 -10.00 2.00 -12 -10.00 2.00
d
4
-6 -9.14 -3.14 -16 -14.57 1.43
d
5
0 0 0 -20 -19.14 0.86
d
6
-4 -4.57 -0.57 -24 -20.00 4.00
d
7
-8 -9.14 -1.14 -18 -19.14-1.14
d
8
-12 -10.00 2.00 -12 -10.00 2.00
d
9
-6 -9.14 -3.14 -6 -9.14 -3.14
d
10
0 0 0 0 0 0

22. Associative: reaction intermediate is
formed by including the incoming
ligand in the coordination sphere
and has higher coordination
number than reactants or products
• Lower coordination number
complexes
• Rates depend on the entering group

26. SYSTE
M
Weak Field / High Spin Strong Field / Low Spin
O
h
OW CFAE O
h
OW CFAE
d
0
0 0 0 0 0 0
d
1
-4 -6.08 -2.08 -4 -6.08 -2.08
d
2
-8 -8.68 -0.68 -8 -8.68 -0.68
d
3
-12 -10.20 1.80 -12 -10.20 1.80
d
4
-6 -8.79 -2.79 -16 -16.26-0.26
D
5
0 0 0 -20 -18.86 1.14
d
6
-4 -6.08 -2.08 -24 -20.37 3.63
d
7
-8 -8.68 -0.68 -18 -18.98-0.98
d
8
-12 -10.20 1.80 -12 -10.20 1.80
d
9
-6 -8.79 -2.79 -6 -8.79 -2.79
d
10
0 0 0 0 0 0

33. It is a continuous single step
process
Two types exist
Interchange associative (I
A
) –
Bond making more important
Interchange dissociative (I
D
) –
Bond breaking more important

36. Ammine complexes of Co(III) are the most
studied.
Water is the medium of reaction.
Usually replacement of NH
3
derivatives is
very slow, so only other ligands are
considered.
[Co(NH
3
)
5
X]
2+
+ H
2
O  [Co(NH
3
)
5
(H
2
O)]
3+
+ X
-
Rate = k. [Co(NH
3
)
5
X]
2+
. [H
2
O]
Rate = k’. [Co(NH
3
)
5
X]
2+

37. Charge on the complex
Steric factors
Effect of leaving group
 Effect of solvent
Presence of pi-donors and acceptors as
spectator ligands

38. The increase in positive charge decreases the rate
of reaction following a dissociative mechanism
because the breaking the metal-ligand bond
becomes difficult.
For aquation of the Ru complexes the trend is as
shown
[RuCl
6
]
3-
1.0 s
-1
[RuCl
3
(H
2
O)
3
]
0
2.1 x 10
-6
s
-
1
[Ru(H
2
O)
5
Cl]
3-
~ 10
-8
s
-1

39. Complex Rate constant
S-1
[Co(NH
3
)
5
(NO
3
)]
2+
~ 10
-5
[Co(NH
3
)
5
I]
2+
~ 10
-6
[Co(NH
3
)
5
F]
2+
~ 10
-8
Thus it is proved that M-X bond
breaking is very much important
in aquation reactions than bond
formation.

40. The rate of aquation of [Co(NH
3
)
5
X]
2+
depends on the
stability of M-X bond.
If the M-X bond is more stable rate of reaction is
low.
The order of reactivity is
HCO
3
-
>NO
3
-
>I
-
>Br
-
>Cl
-
>SO
4
2-
> F
-
>SCN
-
>NO
2
-
This is the order of decreasing thermodynamic
stability of the complexes formed with these groups

41. Anation reactions do not depend very much on the
nature of the entering group, Y
-
.
Instead, it is very much dependent on the nature of the
bond being broken.
 Experimental data show that the rate is of the order 10
-6
for the different entering groups (Y
-
), N
3
-
, SO
4
2-
, Cl
-
or
NCS
-
clearly indicating that the rate is independent of the
nature of the entering group

42. Another important experimental support for
this observation is that ligand exchange
reactions do not take place directly but
instead takes place through aquation and
then anation.
[Co(NH
3
)
5
X]
2+
+ Y
-
[Co(NH
3
)
5
Y]
2+
+ X
-

This indicates that the Co-X bond breaking is very much
significant and then whatever species is present at a higher
concentration will add in anation reaction. Thus, nature of Y
-

is not important

43. When the non-leaving ligands are bulky, they will be
crowding the central metal ion.
The incoming ligand will find it difficult to approach
the central metal ion slowing down the rate of
reaction taking place by associative mechanism.
Instead, if the reaction takes place by dissociative
mechanism, the rate of the reaction will increase
because the crowding around the metal ion is
reduced.

44. • Steric crowding around the metal centre favors
dissociative activation
• Dissociative activation relieves crowding around
the complex
• Steric crowding has been qualitatively and
quantitatively explored
– Tolman Cone Angle

45. Complex k x 10
4
S
-1
Complex k x 10
4

S
-1
Cis-[Co(NH
3
)
4
Cl
2
]
+
Very fast
[Co(NH
3
)
5
Cl]
2+
(0)
4.0
Cis-[Co(en)
2
Cl
2
]
+
150
[Co(en)
2
(NH
3
)Cl]
2+
(2)
0.85
Cis-[Co(trien)Cl
2
]
+
90
[Co(tren)(NH
3
)Cl]
2+
(3)
0.40
trans-[Co(NH
3
)
4
Cl
2
]
+
1100
[Co(en)(dien)
(NH
3
)Cl]
2+
(3)
0.31
trans-[Co(en)(NH
3
)
2
Cl
2
]
+
130
[Co(tetren)Cl]
2+
(4)
0.15
trans-[Co(en)
2
Cl
2
]
+
19

46. AA k x 10
3
min
-1
H
2
N-CH
2
-CH
2
-NH
2
(en) 1.9
H
2
N-CH
2
-CH(CH
3
)-NH
2
(pn) 3.7
H
2
N-CH(CH
3
)-CH(CH
3
)-NH
2
(dl - bn) 8.8
H
2
N-CH(CH
3
)-CH(CH
3
)-NH
2
(m – bn) 250
H
2
N-C(CH
3
)
2
-C(CH
3
)
2
-NH
2
(tetrameen)
instantaneous