Ligand Substitution Reactions:

1
Ligand Substitution Reactions Rates and
Mechanisms
2
Stoichiometric and Intimate Mechanisms

• We can think of a reaction mechanism at two
  different levels.
• The reaction may occur through a series of
  distinct steps each of which can be written as a
  chemical equation.
• This series of steps is a stoichiometric
  mechanism.
• We can also consider what is happening during
  each of these individual steps.
• These details constitute the intimate mechanism
  of the reaction.

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Stoichiometric Mechanism

• Each step in the stoichiometric mechanism has a
  rate or equilibrium constant associated with it.
• The stoichiometric mechanism looks at the
  reactants, products and intermediates that are
  involved in a reaction.
• Each species considered exists in potential
  minimum along the reaction coordinate.

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Stoichiometric mechanism the sequence of
elementary steps in a reaction
5 coordinate intermediate
Dissociative Mechanism, D
5
7 coordinate intermediate
Associative Mechanism, A
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In general, a D mechanism requires evidence for
the existence (structural, spectroscopic) of an
intermediate with reduced coordination number.
An A mechanism requires evidence of an
intermediate with increased coordination number.
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If there is no identifiable intermediate, then we
have to assume an interchange mechanism is
operating
transition state rather than an intermediate
Interchange Mechanism, I
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Intimate mechanism this describes the nature of
the process in the rate-determining step.
If the rate is strongly dependent on the nature
of the entering group, then the intimate
mechanism is associative. We say the reaction is
under associate activation. The symbol is a
subscript a.
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Suppose for the reaction M(NH3)3(OH2)n
Lm- ? M(NH3)3L(n-m) H2O

1. there is spectroscopic evidence for the existence
   of a 5 coordinate intermediate
2. the rate of the reaction is strongly dependent on
   the nature of L (for example, if L H2O the
   reaction occurs 4 orders of magnitude slower than
   if L CN-)

1. tells us that we are dealing with an A
   stoichiometric mechanism
2. tells us that the intimate mechanism is a

The mechanism of the reaction is Aa
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intermediate
rate determining process
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Whilst less common the situation could arise
where the reaction proceeds through an
intermediate of reduced coordination number (D)
and this is followed by rate-determining attach
of entering L on the intermediate (a).
The mechanism would then be described as Da.
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The mechanism would then be described as Da.
Reversible formation of a 5 coordinate
intermediate
Product
Rate-determining attack of entering ligand
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In a Ad reaction, formation of the intermediate
of higher coordination number occurs relatively
rapidly the rate-determining step is the
dissociation of a ligand from the intermediate
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If there is no experimental evidence for an
intermediate, then we have to assume an
interchange, I, mechanism. In this mechanism,
bond breaking and bond making occur
simultaneously and there is no well-defined
intermediate along the reaction coordinate.
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An interchange, I, mechanism could be under
either associative or dissociative activation,
i.e., Ia or Id
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If the rate of the reaction is strongly dependent
on the nature of the entering group and is weakly
dependent on the nature of the leaving group,
then bond making is more important than bond
breaking. The reaction is under associative
activation. We say the mechanism is an
Associative Interchange Mechanism, Ia
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If the rate of the reaction is weakly dependent
on the nature of the entering group and is
strongly dependent on the nature of the leaving
group, then bond breaking is more important than
bond making in the approach to the transition
state. The reaction is under dissociative
activation. We say the mechanism is a
Dissociative Interchange Mechanism, Id
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Ia
Id
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Self-exchange reactions
M(H2O)6 H2O ? M(H2O)5(H2O) H2O
(eg., from line shape analysis using 17O NMR)
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• Rate
• increases with ionic radius
• decreases with an increase in ionic charge

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• Rate
• increases with ionic radius
• decreases with an increase in ionic charge

Inertness ? ?ion
? Self exchange reactions at metal centres are
usually under dissociative activation
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For the transition metals...

• Inertness ? ?ion

• Jahn-Teller distortion of high spin d4 and d9
  complexes imparts on them significant lability.

This is an example of how a ground state
structural effect can influence kinetics
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• There is a strong correlation between Ligand
  Field Stabilisation Energy (LFSE) and inertness

For example, low spin Co3 and Cr3 are amongst
the most inert transition metal ions
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d3 LFSE -12 Dq Cr(III) d6 (LS) -24Dq
2P Co(III) d8 -12Dq Ni(II) d7
(HS) -8Dq Co(II) d9 -6Dq Cu(II) d10 0 Z
n(II)
Expected order of lability Co(III) lt Cr(III)
Ni(II) lt Co(II) lt Cu(II) lt Zn(II)
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Expected order of lability Co(III) lt Cr(III)
Ni(II) lt Co(II) lt Cu(II) lt Zn(II)
Observed order of lability Cr(III) Co(III) lt
Ni(II) lt Co(II) lt Zn(II) lt Cu(II)
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Observed order of lability Cr(III) Co(III) lt
Ni(II) lt Co(II) lt Zn(II) lt Cu(II)
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Hence LFSE (a thermodyamic parameter) is a
rough guide to the rate of self-exchange
reactions at metal centres (a kinetic parameter).
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2nd and 3rd transition series
Usually very inert

• High LFSE
• Strong M-L bonds because of good overlap
  between ligand orbitals and the more expansive
  (compared to 3d) 4d and 5d orbitals

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Clearly the LFSE contributes to the kinetic
behaviour of a metal ion, i.e., there must be a
ligand field contribution to the activation
energy (LFAE)
LFAE LFSETS - LFSEGS
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EXAMPLE
Cr(H2O)63 ? Cr(H2O)5???(H2O)3
LFSEGS -12Dq
LFSETS

• Assumptions
• the reaction is under dissociative activation
• the departing ligand in the TS is far from the
  metal centre, i.e., that the TS is approximately
  5-coordinate

The LFSE of the TS will depend on the geometry of
the TS, and two reasonable geometries can be
envisaged, viz., square pyramidal (C4v) and
trigonal bipyramidal (D3h)
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The LFSE of the TS will depend on the geometry of
the TS, and two reasonable geometries can be
envisaged, viz., square pyramidal (C4v) and
trigonal bipyramidal (D3h)
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Method of Krishnamurthy and Schaap to estimate
LFSE of geometries that are neither Oh nor Td
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Method of Krishnamurthy and Schaap
axial ligand field
equatorial ligand field
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axial
equatorial
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axial
equatorial
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axial
equatorial
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axial
equatorial
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axial
equatorial
Average of 2.93 and -4.57 is -0.82
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axial
equatorial
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axial
equatorial
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LFSETS 2(-2.71) 0.82 -6.24 Dq
LFSEGS -12Dq
LFAE -6.24 (-12) Dq 5.76 Dq
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For Cr(III), Dq 1760 cm-1 (from electronic
spectroscopy), so LFAE 10138 cm-1
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From this kind of approach
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Predicted rate Co(III) lt Cr(III) lt Ni(II) lt
Fe(III) lt Mn(III)
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Predicted rate Co(III) lt Cr(III) lt Fe(III) lt
Ni(II) lt Mn(III)
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Predicted rate Cr(III) lt Mn(III) lt Co(III)
Ni(II) lt Fe(III)
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Experimental rate Cr(III) lt Co(III) lt Fe(III) lt
Ni(II) lt Mn(III)
Hence, probably a D mechanism, possibly with a
C4v intermediate.
There is other evidence to suggest that many
Cr(III) reactions have a distinctly associative
character, explaining the very inert nature of
Cr(III) complexes