1. 1/10/2024 1
Stability of Coordination Compounds
• The kinetic stability depends on the activation energy (DG‡)
of the ligand substitution reaction, the thermodynamic stability
is given by the free energy change:
2. 1/10/2024 2
Mechanisms for Substitution Reactions
If rate determining step is:
a)breaking bond of leaving group –>
dissociative mechanism (D)
(this mechanism corresponds to the SN1 reaction in
organic chemistry)
b) making bond of entering group –>
associative mechanism (A)
(this mechanism corresponds to the SN2 reaction in
organic chemistry)
c) Interchange (I)
• Both, dissociative and associative reaction
mechanisms involve two-step pathways and an
intermediate:
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There are several ways in which a substitution reaction can
take place:
1.Dissociative substitution involves the reversible
dissociation of one ligand to form an intermediate complex
with one fewer ligand followed by the addition another ligand
to the metal.
(a) The rate is first order in MLx and zero order in L'.
(b) The rate doesn't depend on the nucleophilicity of L'.
(c) This is common for octahedral complexes and other
complexes with an 18 electron count.
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2-Associative substitution occurs when the incoming
ligand first adds to the complex to form an intermediate
complex with one lignad more than the starting complex
followed by a loss of one ligand.
(a) The rate is first order in MLx and first order in L'.
(b) The rate is directly proportional to the nucleophilicity of
L'.
(c) This is common for square planar complexes and other
complexes with less than an 18 electron count.
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3-Interchange substitution is intermediate between the two.
There is no intermediate complex formed. Instead bond
making to the new ligand occurs along with bond breaking to
the original ligand. This is very similar to SN2 substitution in
organic chemistry.
(a) The rate is first order in MLx and zero order in L'.
(b) The rate doesn't depend on the nucleophilicity of L'.
(c) The rate is inversely proportional to the strength of the
M-L (leaving group) bond.
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Dissociative: activated state has lower coordination number due to
dissociation of the leaving group
Associative: activated state has a higher coordination number due to
bonding of the incoming group
Rate determining step:
dissociation of X (outgoing group) is slow
Rate determining step:
association of Y (incoming group) is slow
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II.1 The Dissociative Mechanism
•In this mechanism the intermediate has a lower
coordination number than the reactant and this
intermediate stabilizes for long enough to equilibrate with
its environment before capturing the entering group.
•The leaving group has left the reaction centre before any
interaction between the metal centre and the entering
group has taken place.
•The rate of the reaction should therefore be insensitive to
the nature and concentration of the entering group but
dependent on the nature of the leaving group.
•Referring to Figure 4, the reaction profile consists of a
single intermediate and two transition states.
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II.2 The Associative Mechanism
•In the A mechanism the intermediate has a larger coordination
number than the reactant.
•It can be seen in Figure 5 that in the first transition state
(bond making) the bond with the entering group Y is largely
established before the bond towards the leaving ligand X is
weakened.
•The reaction rate therefore depends strongly on the nature of
the entering ligand Y since it participates in the transition state.
•This reaction profile consists of a single intermediate
(containing the entering and leaving group bonded to the
reaction centre) and two transition states leading to the
formation and decomposition of the intermediate.
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(a) the bond-breaking transition state at higher energy (b) the bond-
making transition state at higher energy. the deeper the energy well the
more stable the intermediate will be.
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II.3 The Interchange Mechanism
•In this mechanism no detectable intermediate can be
found and there is only a single transition state.
•Here the acts of bond making and breaking are either
synchronous or else take place within a pre-formed
aggregate.
•The incoming group Y and the leaving group X are
interchanged between the inner and outer coordination
spheres of the metal.
•As the entering group approaches the reaction centre the
leaving group departs.
•ID and IA can be considered as extreme border cases of
the I mechanism.
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III.Factors Affecting Rates of Reactions
1- Effect of Temperature
The effect of temperature on the rate constant of a reaction (k) is
governed by
Where K is Boltzman`s constant
h is Planck`s constant
DH≠ and DS≠ are the standard enthalpy and entropy of activation,
respectively, and represent the change in enthalpy and entropy
respectively, in the formation of the activated complex (transition
state) from the reactant.
DH≠ is related to the energy required in bringing reactants up to
each other and energy required for reorganization of bonds.
DS≠ measures the degree of disorder on the formation of activated
complex.
If DS≠ is +ve, reaction goes by dissociative mechanism.
If DS≠ is -ve, reaction goes by associative mechanism.
k KT h e e
H RT S R
( / ) / /
D D
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2- Effect of External Pressure
The effect of external pressure on the rate of a reaction is
expressed by the relation
Where DV≠ (volume of activation) is the molar volume change
that takes place in the conversion of the reactants to the
activated complex; k2 and k1 are the rate constants at the
pressures P2 and P1 respectively, at a constant temperature, T.
Since DV≠ can have either positive or negative values,
reactions may be accelerated or retarded by increase of
pressure.
DV≠ is diagnostic of the mechanism; as DV≠ is related to DS≠.
For aquatic reactions of octahedral complexes, this following
relationship holds
D D
V S
104 4 4
. .
RT
P
P
V
k
k
T
303
.
2
)
(
)
log( 1
2
1
2
D
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3- Effect of ionic strength
According to Debye-Huckel theory, the rate of a
reaction between ions of charge ZA and ZB should vary
with the ionic strength (I) of the solution in the
following manner
Where k1 and ko are the rate constants at ionic strength
I and 0 respectively and the constant Q is proportional
to the solvent dielectric constant.
log log /
k k QZ Z I
o A B
1
1 2
2
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4- Influence of Solvent
If the interacting ions have ZA and ZB and they are at a
distance dAB apart in the transition state, then it can be
shown that
Where e is the electronic charge, K the Boltzaman`s
constant, T the absolute temperature, k the rate
constant in a medium of dielectric constant and k is
the value of rate constant in a medium of infinite
dielectric constant.
log log
.
k k
Z Z e
d KT
A B
AB
2
2 303
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Kinetic concepts:
Inert complex: An inert complex reacts slowly, even though the
reaction may lead to a more stable, thermodynamically favoured
product.
→ high energy of activation EA
Labile complex: Fast reactions (reaction half-life < 1 min.)
→ low energy of activation EA
• Generally complexes with high LFSE are inert.
Inert and Labile Complexes
Thermodynamic concepts:
Stable complex: Large complex formation
constant
Unstable complex: Small complex
formation constant
e.g. [Cu(OH2)6]+ is stable in water but
unstable against the formation of
[Cu(NH3)4(H2O)2]+ in the presence of NH3
in aqueous solution.
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It was assumed that a stable complex is inert and that an unstable
complex is labile. This is not always true, as cyanide ion forms
very stable complexes with metal ions such as Ni2+. The stability
indicates that the equilibrium (1) lies far to the right and that Ni2+
prefers CN- to H2O as a ligand
[Ni(H2O)6]2+ + 4CN- [Ni(CN)4]2- + 6H2O (1)
However, when 14C-labeled cyanide ion is added to the solution, it
is almost instantaneously incorporated into the complex (2). Thus
the stability of this complex does not ensure internees
[Ni(CN)4]2- + 414CN- [Ni(14CN4)]2- + 4CN- (2)
It is important to know that the terms stability and lability relate to
different phenomena. The stability of a complex depends on the
difference in energy between reactants and products
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A stable compound will be considerably lower in energy than
possible products. The lability of a compound depends on the
difference in energy between the compound and the activated
complex; i.e., if this activation energy is large, the reaction will be
slow.
Reactants
Energy
Products
Activated complex
Activation energy
Reaction energy
Scheme (2) The relative energies of reactants, activated complex and
products of a reaction
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Labile Complexes
1- All complexes in which the central metal atom contains d
electrons in eg orbitals (the dx2-y2 and dz2 orbitals that point
toward the six ligands).
For example:
[Ga(C2O4)3]3-, d10 (t2g
6eg
4); [Co(NH3)6]2+, d7 (t2g
5eg
2);
[Cu(H2O)6]2+, d9 (t2g
6eg
3); [Ni(H2O)6]2+, d8 (t2g
6eg
2);
[Fe(H2O)6]2+, d5 (t2g
3eg
2).
2- All complexes that contain less than three d electrons, for
example,
[Ti(H2O)6]3+, d1; [V(Phen)3]3+, d2; [Ca(EDTA]2-, d0
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• For the main group elements, there appears to be a correlation
between the ion size/charge-density and exchange rate:
• For transition metal elements the ion-size/charge density is less
important and the d-electron configuration largely determines the
rate.
• However, trivalent ions still do react more slowly than the
divalent ones.
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•The rate behaviour of complexes is affected by charge and size of
their central atoms. Small highly charged ions form complexes that
react slowly. Thus there is a decrease in lability with increasing
charge of the central atom for the isoelectronic series
[AlF6]3- > [SiF6]2- > [PF6]- > SF6
•Similarly, the rate of water exchange (1) decreases with increasing
cationic charge in the order:
[Na(H2O)n]+ > [Mg(H2O)n]2+ > [Al(H2O)n]3+
•Complexes having central atoms with small ionic radii react more
slowly than those having larger central ions, for example
[Mg(H2O)6]2+ < [Ca(H2O)6]2+ < [Sr(H2O)6]2+
•For a series of octahedral metal complexes containing the same
ligands, the complexes having central metal ions with the largest
charge-to-radius ratios will react the slowest.
[M(H2O)6]n+ + 6H2O* [M(H2O*)6]n+ + 6H2O (1)
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•In general, four-coordinated complexes (both tetrahedral and
square planar molecules) react more rapidly than analogous six-
coordinated systems. The greater rapidity of reactions of four-
coordinated complexes may be due to the fact that there is enough
room around the central ion for a fifth group to enter the
coordination sphere. The presence of an additional group would aid
in the release of one of the original ligands.
•For square planar complexes it is not possible to apply
successfully the charge-to-radius ratio generalization that works
well for six-coordinated complexes.
•Therefore, the rules that predict rate behaviour for six-coordinated
systems will often not apply to complexes having smaller
coordination numbers. Since rate behaviour is dependent on
mechanism and since reactions of metal complexes are known to
proceed by a variety of paths, it is impossible to make
generalizations that apply to all complexes.
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IV. Mechanisms of Octahedral Substitution
Reactions
A)Dissociative Mechanism
Step 1. Dissociation of X to yield a 5 coordinate intermediate
ML5X + Y → [ML5Y] + X (1)
M-X bond is broken Slow and rate determining The rate of D is
only depends on the conc. of ML5X
k1
ML5X → ML5 + X (2)
Trigonal Bipyramidal D3h Square Pyramidal C4h
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B)Associative Mechanism
Step 1. Collision of ML5X with Y to yield a 7-coordinate
intermediate. (slow)
k1
ML5X + Y → [ML5XY] (slow, rate determining) (3)
Capped Octahedron Pentagonal Bipyramid
L
L
L
L X
M
Y
L
M
L
L L
Y
X
L
L
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IV.2 Replacement of Coordinated Water by other Ligand
The rate of replacement of coordinated water molecule by SO4
2-,
S2O3
2-, EDTA and other species has been measured for a variety of
metal ions (1)
The rates of these reactions are independent on the concentration of
the entering ligand, that is, a first-order rate law. Eq. 2 applies. In
many cases
the rate of reaction (1) for a given metal ion is independent of
whether H2O, SO4
2-, S2O3
2- or EDTA is the entering ligand (L).
This observation and the fact that the rate law does not include the
entering ligand suggest that these reactions occur by a mechanism
in which the slow step is the breaking of a bond between the metal
ion and water. The resulting species would then be expected to
coordinate rapidly with any nearby species.
[ ( ) ] [ ( ) ]
M H O M H O L O (1)
X X
n
2
2
2 1
2
+ L + H
2-
2
Rate k M H O X
n
[ ( )
2 ] (2)
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It was found that the more highly charged hydrated metal ions such
as Al3+ and Sc3+ undergo H2O exchange more slowly than M2+
ions. This suggests that bond breaking is important in the rate-
determining step of these reactions. This evidence suggests that
SN1processes are important in substitution reactions of hydrated
metal ions.
IV.3 Replacement of Ligand (or Anion) by Water Molecule
Theses processes are called aquatic reactions. In general, ammonia
or amines coordinated to cobalt(III) are observed to be replaced so
slowly by water that only the replacement of ligands other than
amines is usually considered. The rates of reactions of the type (1)
have been studied and found to be first order in
the cobalt complex (X can be any of a variety of amines). Since in
aqueous solution the concentration of H2O is always about 55.6 M,
the effect of changes in water concentration of the reaction rate can
not be determined.
(1)
X
+
]
)
[Co(NH
H
+
]
)
(
[ -
3
2
5
3
2
2
5
3
OH
O
X
NH
Co
28. 1/10/2024 28
Therefore, the rate law does not tell us whether H2O is involved in
the rate determining step of the reaction. The decision as to whether
these reactions proceed by an SN2 displacement of X by H2O or by
SN1 dissociation followed by addition of water must be made from
the following evidences:
1-The rate of hydrolysis (replacement of one chloride by water) of
trans-[Co(NH3)4Cl2]+ is approximately 103 times faster than that of
[Co(NH3)5Cl]2+. Increased charge on a complex is explained to
strengthen metal-ligand bonds and hence retard metal-ligand bond
cleavage. It is also expected to attract incoming ligands and aid
displacement reactions. Since a decrease in rate is observed as the
charge on the complex increases, a dissociative (SN1) process
seems to operative.
2-Another evidence resulted from the study of the hydrolysis of a
series of complexes related to trans-[Co(en)2Cl2]. In these
complexes the ethylenediamine is replaced by similar diamines in
which H atoms on C replaced by CH3 groups.
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The complexes containing the substituted diamines react more rapidly
than the ethylenediamine complex. The replacement of H by CH3
increases the bulk of the ligands, consequently this will make it more
difficult for an attacking ligand to approach the metal atom. This steric
crowding should retard an reaction. By crowding the vicinity of the
metal atom with bulky ligands, one enhances a dissociative process,
since the removal of one ligand reduces the crowdness around the metal.
The increase in rate observed when the more bulky ligands were used is
good evidence for the SN1 mechanism.
IV.4 Replacement of an acido group (X-) in a cobalt(III)
complex with a group other than (H2O)
This reaction is illustrated by (1)
It has been observed that this reaction takes place by initial substitution
by solvent H2O with subsequent replacement of water by the new group
Y (2)
(1)
X
+
]
)
[Co(NH
Y
+
]
)
(
[ -
2
5
3
-
2
5
3
Y
X
NH
Co
[Co(NH3)5X]2+ [Co(NH3)5(H2O)]3+
H2O
slow
Y
fast
[Co(NH3)5Y)]2+ (2)
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Therefore, in a number of cobalt(III) reactions the rates of reaction (1)
are the same as the rate of hydrolysis.
Hydroxide ion is uniquely different from other reagents with respect to
its reactivity toward Co(III) amine complexes. It reacts very rapidly (as
much as 106 times faster than H2O) with cobalt(III) amine complexes in
a base hydrolysis reaction (3).
In this reaction, the second order kinetics and the unusually rapid
reaction (3) suggest that OH- is
an exceptionally good nucleophilic reagent toward Co(III) and that the
reaction proceeds through an SN2-type intermediate. However, an
alternative mechanism (5), (6), (7).
[ ( ) ] ) ]
Co NH Cl OH
3 5
2
5
2
+ OH [Co(NH + Cl (3)
-
3
-
(4)
]
[OH
]
)
(
[ -
2
5
3
Cl
NH
Co
k
Rate
[Co(NH3)5Cl]2+ + OH- [Co(NH3)4NH2Cl]+ + H2O (5)
fast
[Co(NH3)4NH2Cl] [Co(NH3)4NH2]2+ + Cl- (6)
slow
[Co(NH3)4NH2] + H2O [Co(NH3)5OH]2+ (7)
fast
31. 1/10/2024 31
will also explain this behaviour. The reaction then proceeds by an
process (6) to give a five-coordinated intermediate which then
reacts with the solvent molecule to give the observed product (7).
This mechanism is supported by considering that if no N-H
hydrogen is present in a Co(III) complex, the complex reacts
slowly with OH-. This certainly suggests that the acid-base
properties of the complex are more important to the rate of reaction
than the nucleophilic properties of OH-.
Substitution reactions of a wide variety of octahedral compounds
have now been studied; where a dissociative type mechanism (SN1)
has most frequently been postulated. This result should not be
surprising. Since six ligands around a central atom leave little room
for adding another group. In a very few examples, evidence for
seven-coordinated intermediate has been presented. Therefore, the
SN2 mechanism can not be discarded for octahedral substitution
