Download Chapter 20: Reactions of Complexes (Mechanisms)

Chapter 20: Reactions of Complexes (Mechanisms)
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Two main types will be covered:
– Ligand substitution
Metal‐centered chemistry
– Redox
– A third type involves reactions at the ligands (metal coordination may enhance or change the reactivity of an (in)organic molecule (e.g. benzene)
Ligand substution reactions are extremely abundant and are important properties influencing the applications of metal complexes for catalysis (chemical and biochemical!)
L' + [[ML6]n+
•
L + [[ML5L']]m+
Which is more stable? Ask yourself: With respect to what?
– Kinetics?
– Labile (fast reaction)
– Inert (slow reaction)
– Thermodynamics?
– Stable
±ΔG!
– Unstable
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Review of Rate Laws and Kinetics (Chem 1051)
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You should review Petrucci 9E Chapter 14 (Kinetics) & Ch 24 (Coord. Chem.)
The generalized rate‐law for any reaction is (where [A] = conc’n of A): •
Dis nguishing the cases when n = 1 and n ≠ 1and solving for A(t): 1st order rates give linear ln A vs. t plots
For nth order rates the plot of A(1‐n) vs. t is linear E g when n = 2:
linear. E.g. when n = 2:
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If [A] can be accurately measured over a wide range of conversions, this is a good way of determining the reaction order of A
d
fd t
i i th
ti
d
fA
Often this is not possible. Another way of determining the reaction orders of individual components of a reaction is the Method of Initial Rates:
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The general rate law
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The rate law for a reaction (formation of compound “A”) with several components is:
•
If we hold concentrations of B, C, D, and all other parameters (T, P, etc.) constant, we can simplify it to include all these in a new rate constant:
,
p y
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k is often called the observed rate constant (k
k’
is often called the observed rate constant (kobs)
If we take the logarithm of r, we get
•
Measuring the initial rate of the reaction as a function of the initial concentration of A, [A]0, and plotting log d[A]/dt vs. log[A]0, gives the reaction order a of reactant A as the slope and log k’ as the y‐intercept
Only the rate determining step may be directly measured by traditional means
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Formation Constants (thermodynamics)
Keq =
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[L][(ML5L')m+]
[L'][(ML6)n+]
= Kf
N.B. the square brackets [ ] here imply concentration of the ion or ligand
If L is a solvent (e.g. an H20 ligand in aqueous solution), the concentration of L, [L] is ignored because its conc’n does not change in dilute solutions
If Kf is large, the incoming ligand binds strongly (stronger than solvent)
If Kf is small, the ligand binds weakly.
Because range of Kf values is huge, they are often expressed as log Kf
More than one ligand may be replaced (e.g. [Ni(OH
More than one ligand
may be replaced (e g [Ni(OH2)6]2+ to [Ni(NH
to [Ni(NH3)6]2+)
There are up to six steps, even if we ignore cis‐trans isomerization, and each step has its own formation constant! Yuck!
But these stepwise formation constants yield information about
But, these stepwise formation constants yield information about structure‐activity relationships (SAR)
It’s often difficult to observe each step individually, and instead we obtain an overall formation constant βn, which includes the n‐steps.
an overall formation constant, β
which includes the n steps
– βn = Kf1Kf2Kf3…Kfn
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Generally, the K
Generally,
the Kf of each step decreases with each ligand
of each step decreases with each ligand swapped (Table 20.2)
swapped (Table 20.2)
If Kf increases for the next ligand added, this implies a major change in the electronic structure of the complex (e.g. on going from high‐spin to low‐spin) and therefore a big increase in LFSE.
and therefore a big increase in LFSE.
[Fe(H2O)6]2+ + 3 bipy → [Fe(bipy)3]2+ + 6 H2O [Fe(bipy)2(H2O)2]2+ (hs d6) vs. [Fe(bipy)3]2+ (ls d6)
Thi
This example also experiences a “chelate
l l
i
“ h l t effect” on the thermodynamics (an ff t”
th th
d
i (
entropic effect, but more on that later)
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Substitution Reactions in Octahedral complexes
•
EExchange one (or more) ligand(s) for another, e.g. the weaker ligand
h
(
) li d( ) f
th
th
k li d H2O for a Of
the stronger σ‐donor NH3: •
The different ligand field strengths mean different ΔO, therefore potentially different colours. Can follow by photometric determination (i.e. UV
different colours. Can follow by photometric
determination (i.e. UV‐vis).
vis).
Which is more stable? [Cu(H2O)6]2+ is stable in water, but unstable w.r.t. formation of ammine complex in presence of aqueous NH3, (i.e. it has a large Kf)
How fast is the reaction (i e is it labile or inert?) An inert complex reacts slowly, How fast is the reaction (i.e. is it labile or inert?) An inert
complex reacts slowly
even if the Kf lies to the right (to the thermodynamically favoured product) and has high activation energy (Ea). Labile complexes have low Ea and react fast (t1/2
< 1 minute). Now need KINETIC classification, not just THERMODYNAMIC.
1 minute). Now need KINETIC classification, not just THERMODYNAMIC.
Can we predict labile vs. inert? Yes! High LFSE complexes are “generally” inert
(some exceptions).
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Ligand Exchange is a Nucleophilic Substitution
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The lability or inertness (or “non‐lability” as S&A insist) of substitution reactions depends on both the metal ion and the ligand (Duh!) and also solvent choice
For d‐block ions:
– Inert: d3; l.s. d4, d5, d6; sq. pl. d8
– Labile: d1; d2; h.s. d4, d5, d6; d7; d9; d10
– Intermediate: weak field d
Intermediate: weak field d8 (e.g. Ni
(e g Ni2+ with weak field ligands)
with weak field ligands)
S‐block ions are labile (except for the smallest, Be2+ and Mg2+)
M(III) f‐block ions are labile
M(II) d‐block ions are more ( ) d bl k i
labile than M(III) ions
4d and 5d metals are more i
inert than 3d congeners
h 3d
Rate depends on nucleophilicity of incoming
li d d bili
ligand, and ability of the M to f h M
create a vacant site
Fig. 20.2 p 496
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Classification of Substitution Mechanisms
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The rate determining step in octahedral complexes can be one of three:
– Dissociate (D), Associative (A) or Interchange (I)
Dissociative: loss of one ligand before new ligand binds, leading to a potentially isolable intermediate
Associative: gain of a ligand leads to a highly coordinated intermediate followed by displacement of a weakly bound ligand
Interchange: No real intermediate. Rate of ligand loss and nucleophilic
attack are nearly identical
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Dissociative Mechanism (D‐type) Typical for 6‐coord. complexes
CO
OC
CO
CO
hν
W
OC
CO
CO
CO
CO
W
CO
PPh3
OC
•
OC
CO
+ CO
CO
W
OC
CO
•
W
OC
CO
OC
•
OC
PPh3
CO
Intermediates can be stabilized by “donor solvents”. Solvents like THF are good Lewis bases that can act as place‐holders giving “solvento complexes”.
Ligand loss may need to be encouraged via an excited state. W is l.s. d6 (inert) so (
)
the reaction is SLOW unless heated or photochemically induced by putting an e‐
into an eg* orbital (i.e. t2g6 → t2g5eg1), which is more labile.
Leaving group effects are important, as well as nucleophile
ff
ll
l
h l strength of new L’
h f
’
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Rate laws for D‐mechanisms
ML5X
k1
k-1
ML5 + Y
ML5 + X
k2
ML5Y
d[ML5]
dt
[ML5] =
rds
= k1[ML5X] - k-11[ML5][X] - k2[ML5][Y] = 0
k1[ML5X]
k-1[X] + k2[Y]
][Y]] =
d[ML
[ 5Y]] = k2[[ML5][
dt
• Since k1 << k2, and the accumulation of the intermediate ML5 may be hampered by the reverse reaction, the rate of the overall reaction will d
depend on the rate of formation of ML
d
th
t ff
ti
f ML5
• Steady‐state approximation assumes the conc’n of ML5 will, therefore, be small (as soon as it is formed, it is attacked by either Y or X)
y
)
• We express the overall rate of reaction in terms of rate of formation of ML5:
Solve for [ML5] and substitute into the rate law for p
g
the formation of the product gives the overall rate law for the reaction.
k2k1[[ML5X][Y]
][ ]
k-1[X] + k2[Y]
**Remember, for this mechanism to be true, the intermediate ML5 must be detectable. Because di t d t ti
direct detection at the low concentrations t th l
t ti
expected is experimentally difficult, there are few clear‐cut D mechanisms. If the intermediate can’t be detected, it would be classified as Id
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Associative Mechanism (A‐type)
Typical for four‐
yp
or five‐coordinate complexes
p
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Most common for four‐ or five‐coordinate complexes (esp. sq. planar)
Already has an empty site! Primarily depends on L’ nucleophilicity.
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NC
CN
2-
Ni
NC
14
CN-
3CN
-CN-
NC
Ni
CN
CN
2-
Ni
CN
NC
NC
14
NC
CN
CN
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For sq. planar late metal complexes (Pd
For
sq planar late metal complexes (PdII, Pt
PtII, Ir
IrI) the rate increases for ) the rate increases for
strong nucleophiles and strong π‐acceptors
H2O < Cl‐ < I‐ < H‐ < PR3 < CO, CN‐
Th f
Therefore good leaving groups are ligands
dl i
li d that are poorer nuc’s
th t
’ leading to l di t
a “trans effect” and “trans influence” (kinetic and thermo, respectively)
More on trans effect and influence later (v. important for stereoselectivity
l di t
leading toward cis
d i vs. trans
t
i
isomers)
)
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Rate laws for A‐mechanisms
• The first step depends on the incoming ligand, Y, and is the rds
k1
• Loss of a ligand in second step is fast
ML5X + Y
ML5XY rds
• Steady‐state approximation assumes the conc’n
St d t t
i ti
th
’ of f
k-1
ML5XY will, therefore, be small
k2
•
We express the overall rate of reaction in terms of ML5XY
ML5Y + X
rate of formation of ML5Y:
d[ML5Y]
dt
=
k2k1[ML5X][Y]
= kobs[ML5X][Y]
k-11 + k2
The separate rate constants can now be combined into a single “observed rate constant”, kobs. As expected the rate law is second order and depends on the concentration of
As expected, the rate law is second order and depends on the concentration of both reactants.
**As with D‐mechanisms, there are very few clear‐cut examples A‐mechanisms in which the intermediate is detectable Most reactions fit between the two extremes following interchange
intermediate is detectable. Most reactions fit between the two extremes, following interchange mechanisms. If the intermediate can’t be detected but there is a dependence on the incoming ligand, it would be classified as Ia
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Interchange Mechanism (I‐type)
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Sort of an associative type but common for six‐coordinate complexes
No detectable intermediate since association and dissociation occur in one step l di t hi h
leading to a high energy transition state
t
iti
t t
There is also a subtle distinction between a dissociative interchange (Id) and an associative interchange (Ia)
Basically, if you can observe an intermediate spectroscopically
Basically
if you can observe an intermediate spectroscopically (depending on (depending on
the rate you could use NMR, UV‐vis considering spectroscopic timescales) the mechanism will not be interchange.
Not easy to nail down the rds so have to vary many conditions and study their Not easy to nail down the rds
so have to vary many conditions and study their
effects on rate (and stereochemistry!)
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Figs. 20.3‐20.5
Dissoc.
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Reaction Profiles
Assoc.
Interchange
How do you determine which mechanism occurs?
A: rate depends strongly on incoming ligand
d
d
l
i
i li d (like S
(lik SN2), stronger nucleophiles
2)
l
hil
give faster rates.
D: rate is independent of the incoming ligand (like SN1). If changing the i
incoming ligand
i li d has little effect on rate, the rds
h li l ff
h d is dissociative.
i di
i i
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Two studies
NH2
A
HN Pt
NH2
NH2
NH2
Cl
-
+ I
HN Pt
I
-
+ Cl
NH2
B
HN Pt
NH2
-
Cl + Br
NH2
HN Pt
Br
+ Cl-
NH2
A is ~102 x faster than B. I‐ is a better nucleophile than Br‐. An Assoc. rds.
The tridentate ligand is called The tridentate ligand
is called “dien”
dien (diethylenetriamine) and because it is a (diethylenetriamine) and because it is a
chelating ligand, it is strongly bound to the metal (more on chelate effect)
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A [Ni(H2O)6]2+ + NH3
[Ni(H2O)5(NH3)]2+ + H2O
B [Ni(H2O)6]2+ + Py
P
[Ni(H2O)5(Py)]
(P )]2+ + H2O
A and B have almost the same rates of reaction despite pyridine being a much stronger nucleophile. The rds must be dissociative.
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Probing the Mechanism
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So, we can have a range of detailed mechanisms
• A
Ia
I
Id
D
Studying the mechanism may involve altering the physical conditions (T, P, solvent) but this may complicate matters
Often, trends can be obtained by “fine‐tuning” the ligand. Most useful are NR3 and PR3 ligands, where R groups are easily changed to vary the nucleophilicity of the donor atom without affecting the sterics too much.
For systems where no intermediate is observable (an I mechanism), the rds may be either a or d (associatively activated or dissociatively activated, see p. 499 in S&A)
Computational chemistry is helpful for postulating the relative energies of possible reaction intermediates and transition states, but of course these hypotheses must be supported by other (even if indirect) methods
Finally, you can never prove that your mechanism is right. You can only disprove it.
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The Chelate Effect
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Chelating ligands (en, bipy, dppe) affect both thermodynamics and kinetics of a reaction
+ thousands more
R2N
NR2
N
R2P
N
If R = Ph, 1,2-bis(diphenylphosphino)ethane,
dppe (or diphos)
If R = Me,
M 1,2-bis(dimethylphosphino)ethane,
1 2 bi (di th l h
hi ) th
dmpe
2,2' bipyridine,
If R = H, ethylenediamine, en
If R = Me,, tetramethylethylenediamine,
y
y
, bipy (or dipy)
TMEDA
[M(NH3)4(en)]2+ + 2 NH3
[M(NH3)6]2+ + en
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PR2
Net increase in product molecules vs. reactants ∴ entropic effect
Consider the following data for formation of [Cd(CH3NH2)4]2+ vs. [Cd(en)2]2+ at 298K 298K
Ligand
-TΔS kJ/mol ΔG° kJ/mol
ΔH° kJ/mol
ΔS° J/mol·K
4 CH3NH2
4 CH
‐57
57.3
3
‐67
67.3
3
+20 1
+20.1
‐37
37.2
2
2 en
‐56.5
+14.1
‐4.2
‐60.7
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Trans effect and Trans influence
Most pronounced in square planar complexes (but found in octahedral also)
Spectator ligands, T, (those not undergoing exchange) trans to the leaving group affect the rate of displacement (trans effect) or strength of the bond (trans influence) of the leaving group, X.
• Strong σ‐donor/π‐acceptor ligands accelerate the rate of substitution
p
g
based on σ and π bondingg
• S&A 4E: p. 503 in the text ranks ligands
• Here is an overall trend from weakest to strongest:
F−, H2O, OH− < NH3 < py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− < SO32−
< PR3, AsR
< PR
AsR3, SR
SR2, CH
CH3− < H
< H−, NO, CO, CN
NO CO CN−, C
C2H4
• The greater the M‐T overlap the stronger the effect (C = cis ligand, Y = nucleophile, T = strong Trans‐effect ligand, X = leaving group
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C
C
C
X
T
Y
M
X
T
M
T
M
Y
Y
C
C
C
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A look at rate constants, k
Strong trans effect
Strong trans effect
Weak trans effect
Very strong trans effect
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Steric Effects
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Trans influence raises the ground state of the initial complex (thermodynamics)
Trans effect makes the leaving group more labile (kinetics)
Ligands cis to leaving group have little influence, except when sterically bulky
Large ligands can prevent Associative reactions and favour Dissociative rds
In square planar systems this slows down the ligand exchange and improves kinetic inertness
PEt3
N
Pt
PEt3
k = 8x10-2 s-1
Cl
PEt3
N
Pt
PEt3 k = 1x10-6 s-1
Cl
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Influence on Ea and G.S. Energy
Ea
Ea
G.S.
E
Ea
E
E
G.S.
G.S.
P
P
P
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