11 - reaction mechanisms

8/14/2019 11 - Reaction Mechanisms

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Reaction mechanisms of Coordination

Compounds

Advanced Inorganic Chemistry


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The Kinetic Model - Transition State

Theory Molecules obtain sufficient energy to

reach a state intermediate between

reactants and products This intermediate state is called the

activated complex

There is then a 50% probability of theactivated complexes decaying to productsor reactants


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Principle of microscopic

reversibility Molecules can proceed along the reaction

coordinate in either direction

It is required that at equilibriumbothforward and reverse reactions proceed at

equal rates along the reaction coordinate


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Consequences

the route of entry for the new ligand must

be just the reverse of that for the loss of the

leaving ligand Insofar as the entering group Y resembles

the leaving group X, the mechanism for

replacement of X by X (exchange) mustresemble replacement of X by Y


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Inert and labile

Inertness is a kinetic stability

As we discussed earlier, it reflect the rate at which

the ligands of the complex exchange places withthose molecules in the outer sphere

Thermodynamic stability is described by n

A complex can be thermodynamically stable

(large n), but kinetically unstable (labile)


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Langford and Gray Classification

Class I; (diffusion controlled) k 108s-1

alkali metals, alkaline earths (except Be2+

and Mg2+), Cd2+, Hg2+, Cr2+, Cu2+, sometrivalent lanthanides

Class II; 104< k < 108s-1

divalent first transition series elements(except V2+, Cr2+, Cu2+), Ti3+, Mg2+, othertrivalent lanthanides


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Langford and Gray

Class III; 1 < k < 104s-1

Be2+, V2+, Al3+, Ga3+, several trivalent

lanthanides

Class IV; 10-6< k < 10-2s-1.

Cr3+, Co3+, Rh3+, Ir3+, Ru2+, Pt2+,


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Lability and electron

configuration Inert configurations

d3, low spin d4, d5, d6

d8is borderline (really only weak field Ni2+

complexes)

Strong field d8complexes are square planar


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Mechanisms for ligand

substitution reacts, Oh systems X- leaving group; Y - entering group

Leaving group usually listed last

e.g.

L5MX + Y L5MY + X


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Two issues

intimate mechanism - whether the main

factor controlling the activation energy is

the breaking of the M-X bond (dissociative,d) or making of the M-Y bond (associative,

a)

stoichiometric mechanism - what is thesequence of elementary steps leading from

reactants to products


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Dissociative (D) mechanism

Bond-breaking most important. Complex is

surrounded by solvent molecules, Y and other

molecules that may be present L5M intermediate must be observed to verify D

L5MX

k1

k-1

L5M + X

L5M + Yk2

L5MY + X


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Id- Interchange dissociative

primary contribution to activation energy is bond-

breaking, but L5M is not detectable

M-X is breaking, but M-Y is starting to form

L5MX + YK

(L5MX,Y)

(L5

MX,Y)k

(L5MY,X)

(L5MY,X)fast

L5MY + X


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Experimental parameters

Can control [Y]o, [L5MX]o

rate = kobs[L5MX]

Note [L5MX]o= [L5MX] + K[L5MX][Y]


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Associative (A) mechanism

No firmly established A mechanisms for

octahedral complexes since seven

coordinate intermediate has never beenobserved conclusively

rate = kobs[L5MX][Y]

D, Id, and Iaall lead to the same rate law, soadditional tests of mechanism are needed


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Experimental tests of mechanism

No mechanism is actually proven

Easiest thing to distinguish is the intimate

mechanism

Most data is know for inert complexes

(CoIII, CrIII, RhIII, IrIII, PtII, NiII)


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Reactions studied - ligand

exchange Aquation (often termed acid hydrolysis)

L5MXn++ H2O L5M(OH2)

(n+1)++ X-

AnationL5M(OH2)

(n+1)++ X-L5MXn++ H2O


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Sensitivity to the Nature of the

Entering or Leaving Group Consider Table 11.3 - rates of aquation of

[Co(NH3)5X]n+

Rates depend heavily on nature of X (sixorders of magnitude)

Relatively little sensitivity to Y

Therefore, breaking M-X bond is more

important


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Sensitivity to the Nature of the

Entering or Leaving Group One problem is that in water, no direct

replacement of X-occurs, rather aquation

followed by anation Conversely, [Ti(OH2)6]

3+shows a rate

variation with entering groups ~ 104(Table

11.5)


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Electronic effects of inert ligands

for cis-[Co(en)2LX]n+(Table 11.6)

When X leaves, the orbital of the d2sp3hybrid isempty

If the cis L is a good -donor, it can supplyelectrons to the electron deficient CoIII.

This stabilizes the transition state and lowers the

activation energy Therefore, cis complexes with good -donors,

react more rapidly than cis complexes with -acceptor ligands (CN-) or -only donors (NH3)


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Electronic effects of inert ligands

with L trans to X, no stabilization can takeplace unless the complex rearranges to

trigonal bipyramidal. this rearrangement raises Eaand lowers the

rate relative to the corresponding ciscomplex

These observations would be hard toreconcile with an a intimate mechanism


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Comparison of rates for Anation

and Water Exchange Since water (or any solvent) concentration cannot

be varied, so its presence in the transition state

cannot be discerned kinetically Can use 18OH2as X

L5M(18OH2) + H2O L5M(OH2) +

18OH2

Mechanism is Id, since K[H2O]>>1, so kobsis

identified with the rate for dissociative interchangeof H2O between the inner and outer sphere


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Comparison of rates for Anation

and Water Exchange Similar limiting rate laws can be observed if [Y] is

high enough

Reaction is then 1st order in L5MX and k

obsis in

units in s-1

at low [Y], 2nd order kinetics are observed

at high [Y], 1st order (saturation) kinetics are

observed plots of kobsvs [Y] will help. Limiting first order

behavior may not be observed if k2or K is toosmall


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Effect of charge on reaction rate

Increased positive charge should make

bond-breaking more difficult

rate would decrease with increasingpositive charge for d intimate mechanism

true for main group elements

for TMI, CFSE overlay the charge effects


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Activation parameters

G= H- TS

H- energy requirements for reaching the

transition state

S- change in ordering on reaching the

transition state

Activation parameters can often revealdifferences masked by similar values of kobs


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Activation parameters

also have V- volumes of activation

kobsexp(-PV/RT)

a plot of ln kobsvs P gives a straight line of

slope -V/RT

Data more complex than expected

V= Vinstrinsic+ Vsolvent(latter term

more important for charged ligands


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V> 0 when anionic ligands leaveanionic complexes

See Table 11.12 for summary

11 - reaction mechanisms

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