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Reaction Mechanisms of Inorganic Compounds
C21J, 4 Lectures
Mohammed Bakir, Office #8
Introduction
Ligand substitution reactions
Octahedral Complexes
Square Planar
Racemization Reactions
Reference: Chapters 9
Textbook: B. Douglas, D. H. McDaniel, & J.
J. Alexander, Concepts & Models of
Inorganic Chemistry, 2nd edition, Wiley,
New York
Some Chemistry of Transition Metals
Ligand Exchange
Photochemical
Coordination to metals
Redox Reactions
Ligand Reactivity
Rearrangement of coordination
Sphere
What are reaction mechanisms?
Detail step-by-step analysis of reaction
pathway.
i.e. sequence of reactions involved in the
conversion of reactants to products
Rate Law
‘Intimate’ Mechanism refers to details of
the mechanism at the molecular level.
Consider the reaction:
MWL
W
Reaction Profile
M + L W ML
W
[ML]*
Collisions
of >Energy
[ML]
Activated Complex
Transition State
M+L
ML
Reaction Coordinate
Potential Energy
Potential Energy
Activation
Energy α ∆G‡
[ML]**
Intermediate
ML
[ML]*
W
Activated Complex
Width related to entropy
Narrow for –∆
∆S
Wide for + ∆S
[ML]* [ML]**
M + L [ML]
ML
Reaction Coordinate
Microscopic Reversibility
Microscopic reversibility: at equilibrium
both forward and reverse reactions
proceed at equal rates backward and
forward and along the reaction coordinate.
Transition State Theory
Rate = k[L][M]
k α K‡ α exp (-∆G‡/RT)
k α exp (∆S‡/R) exp (-∆Εact/RT)
Fast reactions are favored by:
∆S‡ more positive.
∆Eact less positive.
Note since k is exponentially dependent on
∆S ‡ and ∆Eact. Small changes in k are not
significant in the interpretation of the rate
law.
Type of Reactions
Ligand Substitution reactions: are those in which
the coordination sphere around the metal
changes due exchange of one ligand (lewis base)
with another ligand.
e.g.
Octahedral
ML6 + Y W ML5Y + L
[Cu(H2O)6]2+ + conc. HCl W[CuCl4]2seconds
[Cr(H2O)6]3+ + conc. HCl W NO Reaction
Square Planar
ML4 + YW ML3Y + L
fast within
Examples of Substitution Reactions
W
Substitution reaction
[Cu(H2O)4]2+
+NH3
-NH3
W
Cu2+ + H2O
4H2O + [Cu(NH3)4]2+
[Cu(NH3)4]2++HCl+H2O → [Cu(NH3)3(H2O)]2+ + NH4+
H 2O
Fast reactions
[Cu(H2O)4]2+
+ Cl-
[Cu(H2O)3(Cl)]+
Example
[Co(NH3)6]3+ + H2O → [Co(NH3)6(H2O)]3+
Slow several days
Lability and inertness: kinetic terms &
unrelated to thermodynamic stability of
compound and they refer to how quickly a
reaction system reaches equilibrium.
Inertness is kinetic stability
Examples
M(n-y)- + nCN-
W [M(CN) ]
n
y
Kf = [M(CN)n]y/[Mn-y)[CN]n
Compound
Kf
Exchange Rate
[Ni(CN)4]2[Hg(CN)4]2[Fe(CN)6]3[Fe(CN)6]4[Pt(CN)4]2-
1030
1042
1044
1037
~1040
very fast
very fast
very fast
very fast
t1/2 = 1 min.
→ [Hg(CN)4]2- Thermodynamically stable but kinetically
labile, the same for [Ni(CN)4]2i.e. in solutions ligand exchange is fast but always with
CN-.
Thermodynamics versus Kinetics
Thermodynamics
→ Stability
(stable/unstable)→ Kf Formation
Constant
Kinetics →lability and inertness →rate
of replacement of ligands (bases)
Ligand Substitution Reactions in
Octahedral Compounds
Kinetics of H2O exchange
LnM(H2O) + H2O*W LnM(H2O*) + H2O
(see Figure 1)
Kinetics of Water Exchange
Li+Na+K+ Rb+Cs+
Be2+
Al3+
Ln3+
Ru2+
V2+
100
102
Mg2+
Ca2+Sr2+Ba2+
Ln3+Zn2+ Cd2+ Hg2+
Ni2+
104
Co2+ Fe2+ Mn2+
106
108
Exchange rate for metal aqua complexes
Figure 1
Cu2+, Cr2+
1010
Langford & Gray Classes for Water
Exchange
Class I
Very fast (diffusion control), k >108 sec-1. The ions
in this class are those of alkali metals and alkaline earths
(except for Be2+ & Mg2+), Group IIB (except for Zn2+), Cr2+
and Cu2+.
Class II
Exchange-rate constants are between 104-108 sec-1.
The divalent first row transition metal ions (except for V2+, Cr2+
& Cu2+) as well as Mg2+ and the trivalent lanthanide ions.
Class III Exchange-rate constants are between 1 and 104 sec1. This class includes Be2+, V2+, Al3+, Ga3+ and several trivalent
first-row transition metal ions.
Class IV Exchange rate constants between 10-6 to 10-1 sec-1.
Member of the set are Cr3+, Co3+, Rh3+, Ir3+ and Pt2+. Inert.
Factors that affect water exchange
substitution reactions
Ionic potential = q/r; charge & size of metal ions
Ions with high ionic potential → slow reactions
suggesting that the main contribution to the activation
energy is the breaking of bond to the leaving group.
d-electronic configuration
→metal ions in strong LFSE → slow substitution
Oh
dx2-y2
6Dq
dz2
eg
4Dq
dxy
dxz
dyz
t2g
Spectroelectrochemical Series
CN->phen~NO2>en>NH3~py>H2O>C2O42- >OH->F->S2>Cl->Br->IIn strong field metal ions in d3, d4, d5, & d6 are inert
i.e. HIGH LFSE
Oh → high spin d8 such as NiII
& low spin d8 such as Rh(I), Ir(I), Ni(II), Pd(II), Pt(II) &
Square planar Au(III) are labil.
React faster than d6 & d3
For Cu2+ & Cr2+ the fast exchange is due to Jahn-Teller
distortion in the ground state which makes the axial bonds
longer than the equatorial bonds.
Mechanism of Ligand Substitution in Oh
Compounds
k1
ML5X
W
k-1
k2
[ML5] + X
Potential Energy
ML5X + Y → ML5Y + X
Where
X is leaving group
Y is entering group
Consider the following mechanism
Wide ∆S +
ML5X
[ML5] + Y →
ML5Y
ML Y
[ML ]
Dissociative pathway-D-type
Reaction Coordinate
Key: M-X bond breakage, [ML5] is intermediate
5
5
Factors that affect the Strength
of M-X Bond
δ
+
δ
M -X Electrostatic
Charge on metal Center-increase positive charge, M-X
bond strengthen
Size of metal center-increase size of metal center, M-X
bond weaken
Charge of leaving group, increase negative charge on X,
M-X bond strengthen.
Size of leaving ligand, increase size of leaving group, MX bond weaken
Charge on other ligands (spectator), increase negative
on other ligands, M-X bond weaken
Size of spectator ligands, increase size of other ligands,
M-X bond weaken
Rate Law for Dissociative Mechanism
Rate = d[L5MY]/dt =k2[L5M][Y]
Apply the steady-state approximation to L5M
d[L5M]/dt = 0 =k1[L5MX] – k-1[L5M][X] –
k2[L5M][Y]
∴ [L5M] = k1[L5MX]/(k-1[X]+k2[Y])
& Rate = (k1k2[L5MX][Y])/(k-1[X] + k2[Y])
Bimolecular
Dissociative Mechanism, D-type
If k2[y]>>k-1[X]; i.e. pusedo 1st order
∴
Rate = k1[LnMX]
unimolecular
e.g. aquation and anation of inert d6 CoIII
compounds of the type [Co(NH3)5X]2+.
For this mechanism [L5M] is an intermediate that
has an appreciable lifetime and can be
detected.
Note true examples of D-type are not common.
Aquation or acid hydrolysis
Aquation or acid hydrolysis
L5MXn+ + H2O→ [L5M(H2O)]n+1 +XAnation
[L5M(H2O)]n+1 + Y- →L5MYn+ + H2O
∆
-X
L5M…...X
…
L5M-Y
Y
→
Activation Energy
Vibrational Energy
+Y
…..….
L5M-X → L5M—X → L5M….X
Y
The rate is insensitive to the identity of Y,
Implication is that both entering and leaving
groups are weakly bound in the transition
state. This is known as Dissociative interchange
(Id).
No stable intermediate is observed
Plausible Mechanism for Id Dissociative
Interchange
Narrow, ∆S –
L5MX + Y
(L5MX,Y)
W
k2
→
fast
→
(L5MX,Y)
(L5MY,X)
ML5X +Y
[ML5Y,X]
ML5Y
(L5MY,X)
The magnitude of K depends on the ionic charge being
larger when Y and the metal complex are of opposite
charge.
The likelihood of Y present in the outersphere depends
on [Y], as indicated in step 1.
L5MY +X
For short lived
Intermediate, or TS
Potential Energy
K
Reaction Coordinate
Rate Law for Dissociative Interchange
The experimental parameter accessible here is the initial
concentration of the reactant complex [L5MX]0 which in
solution containing Y exists partly in the form of
outersphere complex
[L5MX]0 = [L5MX] + [L5MX,Y]
K = k1/k2 = [L5MX,Y]/[Y][L5MX]
∴ [L5MX,Y] = K[Y][L5MX]
& [L5MX]0 = [L5MX] + K[Y][L5MX]
Hence
[L5MX] = [L5MX]0/(1 + K[Y])
Rate = -dL5MX/dt = k2[L5MX,Y]
Rate = k2K[L5MX]0[Y]/(1 + K[Y])
Associative (a) Mechanism
L5MX + Y
k2
k1
W
k-1
L5MXY
L5MXY → L5MY + X
Rate = k1[L5MX][Y]/(k-1 +k2)
No example for Oh
Associative Interchange (Ia) Mechanism
.
L5M-X →
L
M
—
X
→
L
M
…
.X
5
5
∆
Activation Energy
Vibrational Energy
…
+Y
Y
→
-X
L5M-Y
L5M…...X
Y
Key: M-Y bond starts to form before M-X bond
starts to cleave.
The entering group (Y) is strongly bound, and rate is
sensitive to Y. Implication is that the entering group
and leaving group are strongly bound in the transition
state. This is known as Associative interchange. (Ia)
Summary
Inorganic Reaction Mechanisms
d
a
Ia
SN2(lim)*
Stoichiometric
SN1 & SN2 are not good models for inorganic
reaction mechanisms.*
d
SN1(lim)
Id
SN1
a
SN2
intimate
Summary
L5MX + Y W L5MY + X
Rate = a[L5MX][Y]/(1 + b[Y])
Limiting conditions:
1. b[Y] >>1; ∴ the reaction should be first
order.
2. b[Y]<<1; ∴ the reaction should be second
order.
3. b[Y]=1 ∴ the reaction should be of mixed
order.
A always second order as long as the final step is
not reversible.
Substitution inert ions
Co3+, Cr3+, Rh3+, Ir3+, Pt4+ & Ni2+
Intimate Mechanisms
Nature of leaving & entering groups
L5M-X W L5M + X
For dissociative mechanism rate is dependent
the leaving group
i.e. M-X bond strength
e- - donor σ-donor like NH3, H-,
π-donor→ σ + filled π-orbital
e.g. Br-, O-, Cp-, RO-, RN2 e--acceptor→ σ-donor + empty π*
orbitals e.g. CO
Ligands
σ-donors
H->PR3>SCN->I->CH3-,CO>CN->Br->Cl>NH3>OHπ-acceptors –
CO~C2H4>CN->NO2->NCS->I->Br-
Leaving group order for Oh
compounds
HCO3->> NO3-> I-~ H2O~ Br->Cl-~
SO42-> SCN->F->CH3COO->NCS>NO2->NH3>OH->CN-
Size of the Coordination Sphere &
Metal ION
Large Coordination sphere favors associative
mechanism
Small Coordination sphere favors dissociative dtype pathway.
e.g.
1st → Oh favors D-type
2nd → Oh may be A-type
3rd → Oh favors A-type
Compare water exchange in [Rh(H2O)6]3+ &
[Ir(H2O)6]3+
1:3
Coordination Sphere
Coordinatively saturated
Favors D- type
Coordinatively unsaturated
Favors A-type
Mechanism
1. Leaving group
e.g.
[Co(NH3)5X]2+ +H2O→ [Co(NH3)5(H2O)]3+
[Co(NH3)5(H2O]3++Y- → [Co(NH3)5(Y)]2+
Rate is dependent on leaving group and is
independent on entering group.
i.e. Co-X bond breaking is the rate determining
step.
Evidence?
Aquation or acid hydrolysis
Aquation or acid hydrolysis
L5MXn+ + H2O→ [L5M(H2O)]n+1 +XAnation
[L5M(H2O)]n+1 + Y- →L5MYn+ + H2O
Rate constants for acid aquation of some Oh compounds
of Co(III) of the type [Co(NH3)5(X)]2+ at 25 C
Compound
k (sec-1)
[Co(NH3)5(OP(OCH3)3)]3+
[Co(NH3)5(NO3)]2+
[Co(NH3)5(I)]2+
[Co(NH3)5(H2O)]3+
[Co(NH3)5(Cl)]2+
[Co(NH3)5(SO4)]+
[Co(NH3)5(F)]2+
[Co(NH3)5(N3)]2+
[Co(NH3)5(NCS)]2+
2.5x10-4
2.7x10-5
8.3x10-6
5.8x10-6
1.7x10-6
1.2x10-6
8.6x10-8
2.1x10-9
5.0x10-10
Note: k
depends
heavily
On leaving
group X
∴Confirm
Dissociative
pathway
Limiting Rate constants for anation by Yn- and water
exchange (ke) at 45 C [Co(NH3)5(H2O)]3+
Yn-
k (sec-1)
k/ke
H2O
N3SO42ClNCSH2O
H2PO4-
100x10-6
100x10-6
24x10-6
21x10-6
16x10-6
5.8x10-6
7.7x10-7
1.0
0.24
0.21
0.16
0.13
Rate is insensitive to entering
group
i.e. evidence support Dactivation
Further evidence come from
the substitution studies of Yby XWhere no direct substitution
was
Observed.
Example
[Ru(edta)(H2O)]- + L → [Ru(edta)L]
[Ru(edta)L] + H2O→ [Ru(edta)(H2O)]+ L
Rate constants for substitution by L(kl) of
[Ru(edta)(H2O)]- and for aquation kaq of
[Ru(edta)(H2O)]-at 25ºC
L
Pyridine
NCSCH3CN
kL(M-1sec-1)
6.3x103
2.7x102
3.0x10
kaq(sec-1)
0.061
0.5
3.2
Ligands
COOCOO-
-OOC
N
N
N
COO-
pyridine
edta
Steric Effect of Inert Ligands
Crowding around the metal ion will
retard the rate of reactions that occur
by an a-type mechanism and to speed
up those occurring via a d-type
mechanism.
Steric Effect
N
H
H
N
N
H
H
N
CH3
H3C
N
H
H
N
CH3
N
H
H
N
H3C
CH3
H3C
H3C
N
H
H
N
H3C
CH3
N
H
H
N
CH3
Electronic Effect of Inert Ligands
N
N
- Co
-
L
N
N
+
+
N
π-donation by a cis ligand
into the p component of
an empty d2sp3 hybrid.
L
N
- Co
N
N
π-otbital of trans ligand
is ┴ to d2sp3 hybrid.
+
Rearrangement
N
N
- Co
+
N
+
N
-
L
π-donation from trans Ligand can occur upon rearrangement to
Trogonal bipyramidal
Activation Parameters
∆H‡ Enthalpy of activation is a measure of the
height of the energy barrier, particularly bond
strengths within and between reactants, which
must be overcome to attain the transition state;
If ∆H# <D(M-X), then bond is broken during
activation wher D = dissociation energy.
∆S‡ relates to probability of reaction-it includes
contributions from steric bulkand orientation of
reactants
∆S‡ ~ 0 for unimolecular reactions
+ for dissociative
The more negative the value of
∆S‡ the lower the reaction rate.
Activation Parameters
logk, sec-1
k α ∆G0; more thermodynamically favored reactions proceed faster
Hamond’s postulate: Two consecutively occurring states that have similar energy
along a reaction coordinate will involve only small structural reorganization.
∆G0 = -RTlnK
logK
Slope = 1 for dissociative; i.e. TS or intermediate falls late in the reaction
coordinate; i.e. product-like
Slope ~ 0.5 for associative, TS or [x] falls early in the reaction coordinate;
i.e. reactants-like.
Volume of activation + dissociative & - associative
Activation Parameters, The Eyring Equation
k = RT/Nhe∆S‡/Re-∆H‡/RT
kNh/RT = e∆S‡/Re-∆H‡/RT
keff = e∆S‡/Re-∆H‡/RT
Where N is Avogadro’s Numbern= 6.02x1023 mol-1
H = Plank’s constant = 6.6x10-34 J.S.
lnkeff
Slope = -∆
∆H‡/R
Note : ∆H‡ = Ea -RT
-1
1/T, K
Ligand Substitution Reactions on
Square Planar Complexes
d8 metal ions such as [AuIII, NiII, PdII, PtII,
RhI and IrI ] usually form four coordinate
square planar compounds, especially with
strong ligands.
X
L
M
L
ML3X + Y → ML3Y + X
L
Rate = k1[ML3X] + k2[Y][ML3X]
Mechanism for Sq. Substitution
L
L
M
L
X
+Y
L
Y
Y
L
M
M
L
L
X
L
L
L
L
L
M
L
X
Y
L
M
L
X
tbp
Trigonal bipyramidal may be an intermediate
for A type of TS for Ia
5-coordinate species may be isolated as in
[Ni(CN)5]3- and can be isolated as salt of
[Co(en)3]3+
Reactivity order NiII>PdII>PtII
Y
Nucleophilicity order for square planar
compounds:
CN-~CO>PR3>H->I->Cl->H2O
Leaving group order
H2O>Cl->I->H->PR3>CO~CN-
Rate Law
Rate = (k1 + k2[Y])[ML3X]
Indicate two parallel reaction paths.
In the presence of a large excess of the entering
ligand [Y] remains constant throughout the reaction
Rate = -d[ML3X]/dt = kobs.[ML3X] where kobs. =k1
+k2[Y]
A plot of kobs. versus [Y],
Straight line with k1 = intercept and k2 is the
nucleophilicity strength of Y.
Rates of reaction of
SCN-
kobs = k1 + k2[Y]
kobs, sec-1
I-
C6H5SH
Br-NH2OH
N3NO2-
[Y]
Rate of reaction of trans-[Pt(py)2Cl2 in methanol @ 30 C as a
function of the concentration of entering ligand.
Rate Law
Note all ligands have the same intercept, k1,
implying that they go through the same species.
k2 is different from one ligand to another.
Since the only species present in solution in
addition to the ligands is the solvent molecules
this implies that the solvent is involved in the
first term and the second term in the ligand
participation
Summary
Two pathways are involved:
(1) first order in complex (k1), does not
involve Y in rate determining step. This
mechanistic pathway is solvent mediated.
(2) first order in complex and Y (k2), Y
involved in rate determining step
Mechanism
L
L
K1, slow
L
Pt
X
+S
L
Pt
X
L
YL
L
Pathway #1
S
Pt
L
L
fast
Y
Pathway 2
L
L
k2, slow
L
Pt
X
+Y
Y
L
Pt
X
L
XL
L
Pt
L
L
fast
Y
Summary
Both reactions proceed via an associative
process (A), involving a trigonal bipyramid
transition state. Is there any chemical
justification for this transition state?
(i) Many five coordinate transition metal
complexes are known, e.g., Fe(CO)5,
[CoL2(CO)3] + , [Ni(CN)5]3(ii) ML3X complexes are sterically and
electronically unsaturated and have space for
Y to coordinate.
Evidence
Rate law is consistent with associative
mechanism
Charge on the metal center-has no effect on
the rate of reaction
Steric effect-significant increase in rate was
observed for less hindered ligands, trans- is
faster than cis
C6H5>2Me-C6H4>2,4,6-Me3-C6H2Note trans>cis
For the substitution reaction
Pt(PEt3)2RCl + Y- → Pt(PEt3)2RY + Cl-
Rate law
ML3X
S = solvent
Y
S
ML3Y
Y
ML3S
Schematic representation of parallel
reactions
Effect of Entering & Leaving
Ligands on Rate
Rate constants for ligand displacement in some
square planar Pt(II) compounds
Trans-[Pt(py)2Cl2 + Y → trans[Pt(py)2YCl] + ClY
TC
k2(M-1sec-1)
nPt
CH3OH
CH3OClNH3
N 3ICNPPh3
25
25
30
30
30
30
25
25
2.7x10-7
very slow
4.5x10-4
4.7x10-4
1.55x10-3
1.07x10-1
4.00
249
00
<2.4
3.04
3.07
3.58
5.46
7.14
8.93
Effect of Entering & Leaving
Ligands on Rate
Rate constants for ligand displacement in
some square planar Pt(II) compounds
trans-[Pt(dien)X]+ + py →
trans[Pt(dien)(py)]2+ + X
X
kobs(M-1sec-1)
CN-
SCN
IClH2O
NO3-
1.7x10-8
3.0x10-7
1.0x10-5
3.5x10-5
1.9x10-3
very fast
Factors affecting rate of substitution
1. Nucleophilicity
PtCl2Py2 + Y - → PtClYPy2 + ClStandard Reaction, and methanol as a standard
nucleophile, a base with unshared pair of
electrons such as NH3, I-, Cl-, R-, PR3
nPt = logk2(Y)/k1(CH3OH) = logk2(Y) –
logk1(CH3OH)
Factors Affect Nucleophilicity
Basicity
Oxidation potential of Y
Nucleophilicity order
snPt = logk2(Y)/k2(solvent)
S = nucleophilicity discriminating factor
I - > Br - > Cl - >> F PR3 > AsR3 > SbR3 >> NR3
Sulphur > Oxygen
Soft bases are better nucleophiles for PtII as
it is a soft acid
Trans –ligands & trans effect
"The trans effect is defined as the
effect of a coordinated ligand upon the rate of
substitution of ligands opposite to it.“
For Pt II compounds the order of trans effect
H2O ~ OH - ~ NH3 ~ NR3 < Cl - ~ Br - < SCN ~ I - ~ NO2 - ~ C6H5 - < CH3 - < PR3 ~ AsR3 ~
H - < olefins ~ CO ~ CN-
NH3
NH3
NH3
Pt
Cl-
NH3
NH3
NH3
Pt
Cl
--
NH3
Cl
Pt
Cl
Cl-
NH3
-
NH3
Trans effect in synthesis
Cl
NH3
NH3
Cl
Pt
Cl
Cl
Pt
Cl
Cl
NH3
NH3
Pt
Cl
NH3
Cl
Cl
Try
Synthesis of cis and trans- isomers of
[PtCl2(NH3)(PPh3)] given the reactants
PPh3, NH3 and [PtCl4]2-
Origin of the trans-effect
Electronic (Molecular Orbitals)
σ-donor ligands such as H-, R-, increase
electron density around the metal.
π-acceptor ligands such as CO, CN-, PR3, pback bonding weaken the bond trans to it.
Trans-effect
T.S. Tpy σ-donor π-acceptor
PR3
PR3
M
T
Ground state
X
M
Steric Effect of Inert Ligands
Steric effect is unclear
Trans-Pt(PEt3)2RCl] + y → trans-[Pt(PEt3)2RY] + ClR
C6H5
C6H5
o-C6H4CH3
o-C6H4CH3
2,6-C6H3(CH3)2
2,6-C6H3(CH3)2
Solvent
methanol
DMSO
methanol
dmso
methanol
dmso
Y=CN
3.61
very fast
0.234
3.54
8.49x10-3
3.17x10-2
Y =SC(NH2)2
3.30
0.532
0.652
0.106
4.94x10-2
9.52x10-3
Steric Effect
Steric effect is significant, hints to dissociative
pathway,
i.e. although associative pathway is feasible but
dissociative pathway may become operative in
the presence of bulky Ligands.
Conclusion
(i) 1 st order in substrate and nucleophile,
(ii) k1 (solvent) and k2 (entering ligand)- solvent is
important
A number of factors determine reactivity
(i) Nucleophilicity - different from basicity
(ii) cis and trans effects - kinetic effects, not the
same as trans influence - thermodynamic effect
(iii) Leaving group effect
(iv) Rate depends on the metal M
Predictive synthesis.
Redox Reactions
Electron-transfer
A + B- → A- + B
1. Inner sphere
Transition state involve interpenetrationg coordination sphere or
bridging ligands or interaction between reactants which is
common to both coordination sphere & serves as channel
through which electron flows.
Taube & Meyer Cr2+ & Cr3+
Co(H2O)62+ (Labile) & [Cr(NH3)5Cl]2+ (innert)
Co(H2O)62+
5NH4+
+
H+
2+
[Cr(NH3)5Cl] → [Co(H2O)6]2+ +
[ClCr(H2O)6]2+ +
Inner sphere
[Co(NH3)5Cl]2+ + Cr2+ W [Co(NH3)5ClCr]4+
[Co(NH3)5ClCr]4+ → CrCl2+ + [Co(NH3)5]2+
[Co(NH3)5]2+ + 5H+ W Co2+ + 5NH4+
Cr3+ inert to substitution with
k = 2.9x10-8 M-1sec-1 for Cl- anation of Cr3+
For the reduction k = 6x105 M-1sec-1
∴ Cr-Cl could not have been formed from
substitution of free Cl-
Outer-sphere Electron Transfer
The interaction between the oxidant and
reductant at the time of electron transfer is
small, therefore the coordination shells are
intact; i.e. through space , diffusion control
A
e-
B-
Example Outer-sphere Electron
Transfer
[Fe(me2bipy) 3]2+ + [Fe(bipy) 3]3+ W
[Fe(me2bipy) 3]3+ + [Fe(bipy) 3]2+
Rate = k [Fe(me2bipy) 3]2+ [Fe(bipy) 3]3+
= 108 M-1sec-1
Rate of substitution is ~ 104 M-1sec-1
For outer sphere the redox rate must be
faster than the substitution rate.