Coordination Chemistry IV Reactions and Mechanisms Coordination Compound Reactions Goal is to understand reaction mechanisms Primarily substitution reactions most are rapid CuH 2 O 6 2 ID: 316755
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Slide1
Chapter 12Coordination Chemistry IV
Reactions and MechanismsSlide2
Coordination Compound Reactions
Goal is to understand reaction mechanismsPrimarily substitution reactions, most are rapid Cu(H2O)62+ + 4 NH3
[Cu(NH
3
)
4
(H
2
O)
2
]
2+
+ 4 H
2
O
but some are slow
[Co(NH
3
)
6
]
3+
+ 6 H
3
O
+
[Co(H
2
O)
6
]
3+
+ 6 NH
4
+Slide3
Coordination Compound Reactions
Labile compounds - rapid ligand exchange (reaction half-life of 1 min or less)Inert compounds - slower reactionsLabile/inert labels do not imply stability/instability (inert compounds can be thermodynamically unstable) - these are kinetic effectsIn general:Inert: octahedral d3, low spin d
4
- d
6
, strong field d
8
square planar
Intermediate: weak field d
8
Labile: d
1
, d
2
, high spin d
4
- d
6
, d
7
, d
9
, d
10Slide4
Substitution Mechanisms
Two extremes:Dissociative (D, low coordination number intermediate)Associative (A, high coordination number intermediate)SN1 or SN2 at the extreme limitInterchange - incoming ligand participates in the reaction, but no detectable intermediate
Can have associative (I
a
) or dissociative (I
d
) characteristics
Reactions typically run under conditions of excess incoming ligand
We’ll look briefly at rate laws (details in text), consider primarily octahedral complexesSlide5
Substitution MechanismsSlide6
Substitution Mechanisms
Pictures:Slide7
Substitution MechanismsSlide8
Determining mechanisms
What things would you do to determine the mechanism?Slide9
Dissociation (D) Mechanism
ML5X ML5 + X k1, k-1
ML
5
+ Y
ML
5
Y
k
2
1
st
step is ligand dissociation. Steady-state hypothesis assumes small [ML
5
], intermediate is consumed as fast as it is formed
Rate law suggests intermediate must be observable - no examples known where it can be detected and measured
Thus, dissociation mechanisms are rare - reactions are more likely to follow an interchange-dissociative mechanismSlide10
Interchange Mechanism
ML5X + Y ML5X.Y k1, k–1 ML5
X
.
Y
ML
5
Y + X k
2
RDS
1
st
reaction is a rapid equilibrium between ligand and complex to form ion pair or loosely bonded complex (not a high coordination number). The second step is slow.
Reactions typically run under conditions where [Y] >> [ML
5
X]Slide11
Interchange Mechanism
Reactions typically run under conditions where [Y] >> [ML5X] [M]0 = [ML5X] + [ML5X.Y] [Y]0
[Y]
Both D and I have similar rate laws:
If [Y] is small, both mechanisms are 2
nd
order (rate of D is inversely related to [X])
If [Y] is large, both are 1
st
order in [M]
0
, 0-order in [Y]Slide12
Interchange Mechanism
D and I mechanisms have similar rate laws: Dissociation InterchangeML5X ML
5
+ X
k
1
,
k
-1
ML
5
X + Y
ML
5
X
.
Y k
1, k–1ML5 + Y
ML
5
Y
k
2
ML
5
X.Y ML5Y + X k2 RDSIf [Y] is small, both mechanisms are 2nd order (and rate of D mechanism is inversely related to [X])If [Y] is large, both are 1st order in [M]0, 0-order in [Y]
Slide13
Association (A) Mechanism
ML5X + Y ML5XY k
1
, k
-1
ML
5
XY
ML
5
Y + X k
2
1
st
reaction results in an increased coordination number. 2nd reaction is faster
Rate law is always 2nd order, regardless of [Y]
Very few examples known with detectable intermediateSlide14
Factors affecting rate
Most octahedral reactions have dissociative character, square pyramid intermediateOxidation state of the metal: High oxidation state results in slow ligand exchange[Na(H2O)6]+ > [Mg(H2O)6]
2+
> [Al(H
2
O)
6
]
3
+
Metal Ionic radius: Small ionic radius results in slow ligand exchange (for hard metal ions)
[Sr(H
2
O)
6
]
2+
> [Ca(H
2O)6]2+ > [Mg(H2O)6]2+
For transition metals, Rates decrease down a group
Fe
2+
> Ru
2+
> Os
2+
due to stronger M-L bondingSlide15
Dissociation MechanismSlide16
Evidence: Stabilization Energy and rate of H
2
O exchange.Slide17
Small incoming ligand effect = D or I
d
mechanism
Entering Group EffectsSlide18
Entering Group Effects
Close = I
d
mechanism
Not close = I
a
mechanismSlide19
Activation ParametersSlide20
Ru
II vs. RuIII substitutionSlide21
Conjugate base mechanism: complexes with NH
3
-like or H
2
O ligands,
lose H
+
, ligand trans to deprotonated ligand is more likely to be
lost.
Conjugate Base Mechanism
[Co(NH
3
)
5
X]
2+
+ OH
-
↔ [Co(NH
3
)
4
(NH
2
)X]
+
+ H
2
O (equil)
[Co(NH
3
)
4
(NH
2
)X]
+
[Co(NH
3
)
4
(NH
2
)]
2+
+ X
-
(slow)
[Co(NH
3
)
4
(NH
2
)]
2+
+ H
2
O
[Co(NH
3
)
5
H
2
O]
2+
(fast)Slide22
Conjugate base mechanism: complexes with NR
3
or H
2
O ligands,
lose H
+
, ligand trans to deprotonated ligand is more likely to be
lost.
Conjugate Base MechanismSlide23
Reaction Modeling using Excel ProgrammingSlide24
Associative or I
a
mechanisms, square pyramid intermediate
Pt
2+
is a soft acid. For the substitution reaction
trans
-PtL
2
Cl
2
+ Y
→
trans
-PtL
2
ClY + Cl
–
in CH
3
OH
ligand will affect reaction rate:
PR
3
>CN
–
>SCN
–
>I
–>Br–>N3–
>NO
2
–
>py>NH
3
~Cl
–
>CH
3
OH
Leaving group (X) also has effect on rate: hard ligands are
lost easily (NO
3
–
, Cl
–
) soft ligands with
electron density
are not (CN
–
, NO
2
–
)
Square planar reactionsSlide25
Trans effect
In square planar Pt(II) compounds, ligands
trans
to Cl are more easily replaced than others such as ammonia
Cl has a stronger
trans effect
than ammonia (but Cl
–
is a more labile ligand than NH
3
)
CN
–
~ CO > PH
3
> NO
2
–
> I
–
> Br
–
> Cl
–
> NH
3
> OH
–
> H
2
O
Pt(NH
3
)
4
2+
+ 2 Cl
–
PtCl
4
2–
+ 2 NH
3
Sigma bonding - if Pt-T is strong, Pt-X is weaker (ligands share metal d-orbitals in sigma bonds)
Pi bonding - strong pi-acceptor ligands weaken P-X bond
Predictions not exactSlide26
Trans Effect:Slide27
Trans Effect: First steps random loss of py or NH
3Slide28
Trans Effect:Slide29
Electron Transfer Reactions
Inner vs. Outer Sphere Electron TransferSlide30
Outer Sphere Electron Transfer Reactions
Rates Vary Greatly Despite Same MechanismSlide31
Nature of Outer Sphere Activation BarrierSlide32
Nature of Outer Sphere Activation BarrierSlide33
Inner Sphere Electron Transfer
Co(NH
3
)
5
Cl
2+
+ Cr(H
2
O)
6
2+
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
+ H
2
O
Co(III) Cr(II) Co(III) Cr(II)
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
Co(III) Cr(II) Co(II) Cr(III)
H
2
O + (NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
(NH
3
)
5
Co(H
2
O)
2+
+ (Cl)Cr(H
2
O)
5
2+Slide34
Inner Sphere Electron Transfer
Co(NH
3
)
5
Cl
2+
+ Cr(H
2
O)
6
2+
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
+ H
2
O
Co(III) Cr(II) Co(III) Cr(II)
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
(NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
Co(III) Cr(II) Co(II) Cr(III)
H
2
O + (NH
3
)
5
Co-Cl-Cr(H
2
O)
5
4+
(NH
3
)
5
Co(H
2
O)
2+
+ (Cl)Cr(H
2
O)
5
2+
Nature of Activation Energy:
Key Evidence for Inner Sphere Mechanism:Slide35
Example
[Co
II
(CN)
5
]
3-
+ Co
III
(NH
3
)
5
X
2+
Products
Those with bridging ligands give product [Co(CN)
5
X]
2+
.