rule of playbh.knu.ac.kr/~leehi/index.files/coordination_chemistry... · 2018. 11. 25. ·...
TRANSCRIPT
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Coordination Chemistry IV: Reactions and Mechanisms 12
Rule of play ??
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Background
Studying chemical kinetics Understanding reaction mechanism Finding ways of facilitating the reaction
tC
tABAkRate nm
∆∆
=∆
∆−==
][][][][A + B → C + D k = rate constant
Requirements of reaction mechanism 1. Sum of elementary reactions must be the balanced overall reaction. 2. Reaction mechanism should explain the experimentally-determined rate law.
NO2(g)+CO(g) → NO(g)+CO2(g)
Experimentally, 22 ][NOkRate =
Possible mechanism
NO3(g)+CO(g) → NO2(g)+CO2(g)
NO2(g)+NO2(g) → NO3(g)+NO(g)
221
3 ][][ NOkt
NORate =∆
∆=
rate-determining step
NO2(g)+CO(g) → NO(g)+CO2(g)
221
2 ][][][ NOkt
COt
NORate =∆
∆=
∆∆
=
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Background
Principle of microscopic reversibility: • In a reversible reaction, the mechanism in one direction is exactly the reverse of the mechanism in the other direction. This does not apply to reactions that begin with a photochemical excitation.
•The path between the reactants and the products is always the lowest energy pathway and must be the same regardless of the direction of the reaction.
출발
도착
transition state
intermediate
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Background
Parameters obtained from the kinetic experiments: •Order of the reaction, Rate constant •Free energy of activation, Enthalpy (or heat) of activation, Entropy of activation •Volume of activation
tC
tABAkRate nm
∆∆
=∆
∆−==
][][][][
A + B → C + D
k = rate constant
•Sometimes, we use steady-state approximation for deriving the rate law.
The concentration of the intermediate is assumed to be small and essentially unchanging during much of the reaction
Understanding the mechanism
transition state
intermediate
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Background
Reaction types of transition metal complexes • Substitution reactions
• Dissociation reactions
• Addition reactions
• Oxidation-reduction reactions
• Reactions of coordinated ligands
B
A C
D
X
EB
A C
D
Y
E
+Y+ X
B
A C
D
X
EB
A C
DE
-X+ X
B
A C
DE
B
A C
D
Y
E
+Y
M2+
B
A C
D
X
E
M3+
B
A C
D
Y
E
+ M3+
B
A CH3
D
X
E
M2+
B
A C
D
Y
E
+
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Substitution Reactions Inert and Labile Compounds
Examples of substitution reactions
[Ni(H2O)6]2+ + 6 NH3 [Ni(NH3)6]2+ + 6 H2O green blue
Successive addition of HNO3, NaCl, H3PO4, KSCN, NaF to aq. soln of Fe(NO3)3•9H2O
These reactions are fast. Complexes, which react rapidly, are called labile. Taube’s criterion – t1/2 ≤ 1 min Labile ◄► Inert (Robust) <== kinetic term
Thermodynamically unstable, but kinetically inert
[Fe(H2O)5(F)]2+ : thermodynamically stable, but kinetically labile
Be careful! : thermodynamically ____, kinetically ______
• Inert compound is easier to be studied.
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Substitution Reactions Inert and Labile Compounds
These reactions are fast. Complexes, which react rapidly, are called labile. Taube’s criterion – t1/2 ≤ 1 min Labile ◄► Inert (Robust) <== kinetic term
[Fe(H2O)5(F)]2+ : thermodynamically stable, but kinetically labile
Be careful! : thermodynamically ____, kinetically ______
In general, those with higher LFSE are inert.
Werner was lucky because his compounds were Co(III), Cr(III), Pt(II) which are inert. In other words, he was careful for choosing his complexes.
• Inert compound is easier to be studied.
Thermodynamically unstable, but kinetically inert
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Substitution Reactions Mechanisms of Substitution
Dissociative (D) Mechanism MLnX → MLn + X → MLnY + X
The departing ligand leaves, and a discernable intermediate with a lower coordination number is formed. Rate is independent of Y, and is determined by the breaking of the M-X bond (analogous to SN1).
Associative (A) Mechanism MLnX + Y → MLnXY → MLnY + X The incoming ligand adds to the complex, and an intermediate with a increased coordination number is formed. (analogous to SN2).
Interchange (I) Mechanism MLnX + Y → MLnY + X Y---MLn---X
The incoming ligand is presumed to assist in the reaction, but no detectable intermediates appear. dissociative interchange (Id) and associative interchange (Ia)
Y---MLn---X
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Substitution Reactions Kinetic Aspects of Reaction Pathways
Dissociative (D) Mechanism MLnX → MLn + X → MLnY + X
The departing ligand leaves, and a discernable intermediate with a lower coordination number is formed. Rate is independent of Y, and is determined by the breaking of the M-X bond (analogous to SN1).
dtd ]ML[ 5
Mechanism
Steady-state approximation
]ML[ 5
Rate law
[Y][X]X][Y][ML][Y][MLY][ML
21
51252
5
kkkkk
dtd
+==
−
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Substitution Reactions Kinetic Aspects of Reaction Pathways
Associative (A) Mechanism MLnX + Y → MLnXY → MLnY + X The incoming ligand adds to the complex, and an intermediate with a increased coordination number is formed. (analogous to SN2).
Mechanism rate determining step
Steady-state approximation
21
515
5251515
][Y]XLM[]XYLM[
0]XYLM[]XYLM[][Y]XLM[]XYLM[
kkk
kkkdt
d
+=
=−−=
−
−
Rate law
][Y]XLM[][Y]XLM[XY][MLY][ML5
21
52152
5 kkk
kkkdt
d=
+==
−
2nd order
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Substitution Reactions Kinetic Aspects of Reaction Pathways
Interchange (I) Mechanism MLnX + Y → MLnY + X Y---MLn---X
The incoming ligand is presumed to assist in the reaction, but no detectable intermediates appear. dissociative interchange (Id) and associative interchange (Ia)
Y---MLn---X
Rate law 1
Mechanism 1
Rate law 2
Mechanism 2
or
k1[ML5X][Y] - k-1[ML5Y][X]
if [X] and [Y] are large
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01
0021
1
20
1
1
0021
1
2011
002152
5
][Y1][YM][
][Y1
][YM][
][Y][YM][Y]X[MLY][ML
KkK
kk
kk
kkk
kkkkkk
dtd
+≅
++=
++=⋅=
−−
−
−
Rate law
Substitution Reactions Kinetic Aspects of Reaction Pathways
Interchange (I) Mechanism MLnX + Y → MLnY + X Y---MLn---X
The incoming ligand is presumed to assist in the reaction, but no detectable intermediates appear. dissociative interchange (Id) and associative interchange (Ia)
Y---MLn---X
Mechanism A rapid equilibrium between the incoming ligand and the 6-coordinate reactant to form an ion pair or preassociation complex
1
11
−
=kkK
12 −<< kkSteady-state approximation
0]YXLM[]YXLM[][Y]XLM[]YXLM[525151
5 =⋅−⋅−=⋅
− kkkdt
dIf [Y]0 >> [M]0 (= [ML5X]0) and [ML5Y] is very small. [Y][Y] and ]YXLM[]XLM[M][ 0550 ≅⋅+=
0]YXLM[]YXLM[][Y])YXLM[-M]([]YXLM[52510501
5 =⋅−⋅−⋅=⋅
− kkkdt
d
2011
0015 ][Y
][YM][]YXLM[kkk
k++
=⋅−
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Substitution Reactions Kinetic Aspects of Reaction Pathways
Interchange (I) Mechanism (Preassociation Complex)
Dissociative (D) Mechanism Associative (A) Mechanism
[Y][X]X][Y][MLY][ML
21
5125
kkkk
dtd
+=
− 21
5215 ][Y]XLM[Y][MLkk
kkdt
d+
=−01
00215
][Y1][YM][Y][ML
KkK
dtd
+=
[Y]'[X]X][Y][MLY][ML 55
kk
dtd
+= ][Y]XLM[Y][ML
55 k
dtd
=0
005
][Y'1][YM][Y][ML
kk
dtd
+=
When [Y] is low [X]
X][Y][MLY][ML 55 kdt
d= ][Y]XLM[Y][ML
55 k
dtd
=005 ][YM][Y][ML k
dtd
=
1st order for M, 1st order for Y 1st order for M, 1st order for Y 1st order for M, 1st order for Y
When [Y] is high X][ML"Y][ML5
5 kdt
d= 0
5 ][M"Y][ML k
dtd
= ][Y]XLM[Y][ML5
5 kdt
d=
1st order for M 1st order for M 1st order for M, 1st order for Y
There is no clear-cut to distinguish the mechanisms from the kinetic aspects. Need more experimental evidences to distinguish
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 1. Correlation between inert, labile complexes and Ligand Field Activation Energy (LFAE)
LFAE: Difference in LFSEs of the octahedral reactant and the presumed 5-coordinate transition state
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 1. Correlation between inert, labile complexes and Ligand Field Activation Energy (LFAE)
LFAE: Difference in LFSEs of the octahedral reactant and the presumed 5-coordinate transition state
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 2. Oxidation state of the central ion
Rate
Evidence 3. Ionic radius
Rate
Evidence 4. Decreasing negative charge or increasing positive charge on the reactant compound decreases the rate of substitution. Evidence 5. Steric crowding on the reactant complex increases the rate of ligand dissociation. Evidence 6. Activation energies and entropies are consistent with dissociation. And volume of activation (ΔVact) is positive.
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 7. The rate of reaction changes only slightly with changes in the incoming ligand.
Not much effect of incoming ligand
Dissociative (D) Mechanism Associative (A) Mechanism
[Y][X]X][Y][MLY][ML
21
5125
kkkk
dtd
+=
− 21
5215 ][Y]XLM[Y][MLkk
kkdt
d+
=−
When [Y] is high X][ML"Y][ML5
5 kdt
d= ][Y]XLM[Y][ML
55 k
dtd
=
1st order for M 1st order for M, 1st order for Y
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 7. The rate of reaction changes only slightly with changes in the incoming ligand.
Interchange (I) Mechanism (Preassociation Complex)
01
00215
][Y1][YM][Y][ML
KkK
dtd
+=
When [Y] is high 05 ][M"Y][ML k
dtd
=
1st order for M
On the other hand
K1
k2
Incoming ligand: not much effect on k2 significant effect on K1
Predissociation complex mechanism
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Dissociative (D) Mechanism : Most substitution reactions are dissociative.
Evidence 8. Linear Free-Energy Relationship (LFER)
metal-ligand bond strength dissociation rate of the ligand equilibrium constant (thermodynamic function) rate constant (kinetic function)
RS
RTHK
RTEAk a
00
lnlnln ∆+
∆−=⇔−=
F-
H2PO4-
Cl- Br-
I-
NO3- Indirect evidence for strong influence of the
thermodynamic parameters, ∆H, on the activation energy, Ea. A stronger bond between the metal and the leaving group results in a larger activation energy.
Langford argued a complete dissociation of X-.
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Substitution Reactions Experimental Evidences in Octahedral Substitution
Associative (A) Mechanism : Associative substitution is less common.
Ia Id
Ia
Id
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Substitution Reactions
Conjugate Base Mechanism (SN1CB): A reaction pathway by which many metal amine complexes undergo substitution. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base to give the corresponding polyamine metal hydroxide. Second-order rate law.
])[OHX])[Co(NH( -253
+= kRate
Experimental Evidences in Octahedral Substitution
Conjugate base mechanism (Dissociative !!)
looks like associative, but …
H2O + OH- OH- + H2O Conjugate acid-base
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Substitution Reactions
Conjugate Base Mechanism (SN1CB): ])[OHX])[Co(NH( -2
53+= kRate
Experimental Evidences in Octahedral Substitution
Experimental observations supporting SN1CB: 1. Base-catalyzed exchange of hydrogen from the amine groups takes place under the same conditions as
these reactions. 2. The isotope ratio (18O/16O) in the product in 18O-enriched water is the same as that in the water
regardless of the leaving group. [If associative (incoming water has big influence), the isotope ratio (18O/16O) in the product would be higher. Because equilibrium isotope fractionation for
is K = 1.042 . ???] 3. RNH2 (R = alkyl) complexes react faster than NH3 complexes. (steric crowding) 4. A plot of ln(kOH) versus ln(KOH) is linear regardless of the leaving group. (LFER) 5. When substituted amines with no protons are used, the reaction is very slow or nonexistent.
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Substitution Reactions
Conjugate Base Mechanism (SN1CB): A reaction pathway by which many metal amine complexes undergo substitution. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base to give the corresponding polyamine metal hydroxide. Second-order rate law.
Experimental Evidences in Octahedral Substitution
Base-hydrolysis of [Co(III)(tren)(NH3)Cl]2+
If structurally allowed, the position trans to the leaving group is usually deprotonated. (a) is faster (x104) than (b). ((b) needs
rearrangement)
Base catalyzed hydrolysis of [Co(III)L]
Pseudo-aminate mechanism through deprotonation of
methylene groups
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Substitution Reactions
Kinetic Chelate Effect: Substitution for a chelate ligand is generally slower reaction than that for a similar monodentate ligand. (20 ~ 105 times slower)
Experimental Evidences in Octahedral Substitution
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Substitution Reactions Stereochemistry
Reactions with dissociative mechanisms are more likely to result in random isomerization or racemization and associative mechanisms are more likely to result in single-product reactions; however, the evidence is much less clear-cut.
L L
* [ % cis products = sum of two optical isomers ]
* [ % cis products = sum of two optical isomers ]
L
∆ L
Sterochemistry depends on many factors such as mechanism, reaction rate, concentration, and temperature.
(Dissociative)
(Dissociative or SN1CB)
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Substitution Reactions Stereochemistry
Mechanical aspect: Substitution in trans Complexes
Dissociation mechanism and stereochemical change (a) sq. py. intermediate: retention of
geometry (b) tr. bipy. intermediate, eq. B: (c) tr. bipy. intermediate, ax. B:
Statistical expectation(through tr. bipy) trans trans 1/3 trans cis 2/3 (overall) trans trans ? trans cis ?
Less likely because of bigger rearrangement than in eq. B
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Substitution Reactions Stereochemistry
Mechanical aspect: Substitution in cis Complexes
Dissociation mechanism and stereochemical change (a) sq. py. intermediate: retention of
configuration (b) tr. bipy. intermediate, eq. B: (c) tr. bipy. intermediate, ax. B:
Less likely because of bigger rearrangement than in eq. B
Statistical expectation (through tr. bipy) cis (∆) cis(∆) 2/3 cis (∆) trans 1/3 (overall) cis (∆) cis(∆) ? cis (∆) trans ?
Acid hydrolysis: square pyramidal intermediate pathway
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Substitution Reactions Stereochemistry
Example of concentration effect
Base hydrolysis of Λ-cis-[Co(en)2Cl2]+ (SN1CB) (a) In dilute OH- : retention of configuration (sq. py intermediate) (b) In concentrated OH- : inversion of configuration (tr. bipy intermediate)
Example of temperature effect [Co(en)2Cl2]+ + 2NH3 → [Co(en)2(NH3)2]+ + 2Cl-
(a) T < -33oC : inversion of configuration (in liquid ammonia) (b) T > 25oC : retention of configuration (in liquid ammonia, alcohol solution, or gaseous ammonia)
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Substitution Reactions Stereochemistry
Isomerization of Chelate Rings (Pseudorotation)
Strictly speaking, this is not a substitution reaction. Dissociation of one of the donor atom => Rearrangement => Association of the dissociated atom
X
isomerization
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Substitution Reactions Stereochemistry
Isomerization of Chelate Rings
Strictly speaking, this is not a substitution reaction. Dissociation of one of the donor atom => Rearrangement => Association of the dissociated atom
Trigonal twist (Bailar twist) Tetragonal twist with perpendicular rings
Tetragonal twist with parallel rings α−β isomerization
Pseudorotation
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Substitution Reactions Square-Planar Complexes
Kinetics and Stereochemistry
X][Y][MLX][ML 3231 kkRate +=Experimentally observed
Direct substitution (k2)
Solvent-assisted substitution (k1)
slow (and [S] >> [M]) 1st order fast
Ia
Ia
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Substitution Reactions Square-Planar Complexes
Evidence for Associative Mechanism
1. Many five coordinate transition metal complexes with trigonal bi-pyramidal geometry are known, e.g., Fe(CO)5, [Pt(SnCl)3]+, [Ni(CN)5]3-.
2. Rate law is consistent with associative mechanism.
X][MLX][Y])[ML(X][Y][MLX][ML 33213231 kkkkkRate =+=+=
X][MLX][Y])[ML(X][Y][MLX][ML 3333 kkkkkRate YsYs =+=+=
[Y]Ys kkk +=
trans-[Pt(py)2Cl2]2+ in MeOH
sk
Rate constant for reaction with Y
Rate constant for reaction with solvent
Strong support for two associative pathways
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Substitution Reactions Square-Planar Complexes
Evidence for Associative Mechanism
2. Rate law is consistent with associative mechanism. [Y]Ys kkk +=
Rate constant for reaction with Y
Rate constant for reaction with solvent
Ptslog η=s
Y
kk Swain-Scott equation
Nucleophilic discrimination factor
(for complex)
Nucleophilic reactivity constant (for Y) : nucleophilicity
Reactivity toward to Pt(II)
Reverse order
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Substitution Reactions Square-Planar Complexes
The Trans Effect The effect of a coordinated ligand upon the rate of substitution of ligands opposite to it
For Pt(II) complexes, the order of the trans effect is generally: kinetic effect !!
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Substitution Reactions Square-Planar Complexes
The Trans Effect [Explanation]
1. σ-bonding effect Good σ-bases (e.g., alkyl-, H-) are donating electron density to the metal d orbital which is shared with the leaving group. This weakens the M-X bond in the ground state. (Thermodynamic effect)
trans influence
trans influence
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Substitution Reactions Square-Planar Complexes
The Trans Effect [Explanation]
2. π-bonding effect
•The simultaneous coordination of X and Y in the 5-coordinate transition increase of electron density at the metal •π-back donation to the trans ligand decrease of electron density at the metal during the transition state. stabilizing the transition state (Kinetic effect)
σ-bonding effect + π-bonding effect
trans influence π-bonding effect
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Oxidation-Reduction Reactions
Inner-sphere reaction - include atom transfer processes - a ligand is shared in a bridged intermediate activated complex
1. Substitution reaction followed by linking the oxidant and reductant
[Co(NH3)5(Cl)]2+ + [Cr(H2O)6]2+ → [(NH3)5Co(Cl)Cr(H2O)5]4+ + H2O III II III II
2. Electron transfer
[(NH3)5Co(Cl)Cr(H2O)5]4+ → [(NH3)5Co(Cl)Cr(H2O)5]4+ III II II III
3. Separation [(NH3)5Co(Cl)Cr(H2O)5]4+ + H2O → [(NH3)5Co(H2O)]2+ + [(Cl)Cr(H2O)5]2+
II III II III
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Oxidation-Reduction Reactions
Outer-sphere reaction - simple electron transfer without ligand sharing - non labile ligand spheres - faster than ligand substitution
The rates show very large differences depending on the ability of the electrons to tunnel through the ligands.
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Oxidation-Reduction Reactions
Inner-sphere and outer-sphere reactions
outer
Usually inner Likely outer Usually outer
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Oxidation-Reduction Reactions
Inner-sphere and outer-sphere reactions
outer
Inner (products [Co(CN)5(X)]3- are found)
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Oxidation-Reduction Reactions
Condition for High and Low Oxidation numbers
The overall stability of complexes with different charges on the metal ion depends on LFSE, bonding energy of ligands, redox properties of ligands
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Reactions of Coordinated Ligands
The properties of organic ligands changes significantly by coordination to metal ion. Slow reactions in organic conditions can be promoted in inorganic conditions.
Hydrolysis of Esters, Amides, and Peptides
Peptide hydrolysis by [Co(trien)(H2O)(OH)]2+
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Reactions of Coordinated Ligands
Template Reactions – Formation of a complex places the ligands in the correct geometry for reaction
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Reactions of Coordinated Ligands
Template Reactions – Formation of a complex places the ligands in the correct geometry for reaction
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Reactions of Coordinated Ligands
Template Reactions – Formation of a complex places the ligands in the correct geometry for reaction
w/ metal ion
w/o metal ion
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Reactions of Coordinated Ligands
Electrophilic Substitution
Metal coordination of acetylacetone ⇒ Forcing enol form of the ligand, preventing reactions on the oxygens, concentrating
negative charge on the central carbon ⇒ Making electrophilic substitution easier (similar to electrophilic aromatic substitution)