Principal mechanisms of ligand exchange in octahedral complexes

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Principal mechanisms of ligand exchange in
octahedral complexes
Dissociative
Associative
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Dissociative pathway (5-coordinated intermediate)
Associative pathway (7-coordinated intermediate)
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Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L
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Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L
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Inert and labile complexes Some common
thermodynamic and kinetic profiles
Exothermic (favored, large K) Large Ea, slow
reaction Stable intermediate
Exothermic (favored, large K) Large Ea, slow
reaction
Endothermic (disfavored, small K) Small Ea, fast
reaction
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Labile or inert?
LFAE LFSE(sq pyr) - LFSE(oct)
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Why are some configurations inert and some are
labile?
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Substitution reactions in square-planar
complexes the trans effect
(the ability of T to labilize X)
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Synthetic applications of the trans effect
Cl- gt NH3, py
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Mechanisms of ligand exchange reactions in square
planar complexes
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Electron transfer (redox) reactions
-1e (oxidation)
1e (reduction)
Very fast reactions (much faster than ligand
exchange) May involve ligand exchange or
not Very important in biological processes
(metalloenzymes)
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Outer sphere mechanism
Fe(CN)63- IrCl63-
Fe(CN)64- IrCl62-
Co(NH3)5Cl Ru(NH3)63
Co(NH3)5Cl2 Ru(NH3)62
Reactions ca. 100 times faster than ligand
exchange (coordination spheres remain the
same) r k AB
Tunneling mechanism
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Inner sphere mechanism
Co(NH3)5Cl)2Cr(H2O)62
Co(NH3)5Cl)2 Cr(H2O)62
CoIII(NH3)5(m-Cl)CrII(H2O)64
Co(NH3)5Cl)2Cr(H2O)62
CoII(NH3)5(m-Cl)CrIII(H2O)64
CoIII(NH3)5(m-Cl)CrII(H2O)64
CoII(NH3)5(m-Cl)CrIII(H2O)64
CoII(NH3)5(H2O)2 CrIII(H2O)5Cl2
CoII(NH3)5(H2O)2
Co(H2O)62 5NH4
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Inner sphere mechanism
Reactions much faster than outer sphere electron
transfer (bridging ligand often exchanged) r
k Ox-XRed k (k1k3/k2 k3)
Tunneling through bridge mechanism
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Brooklyn College Chem 76/76.1/710G Advanced
Inorganic Chemistry(Spring 2008)
Unit 6 Organometallic Chemistry
Part 1 General Principles
Suggested reading Miessler/Tarr Chapters 13 and
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Elements of organometallic chemistry
Complexes containing M-C bonds Complexes with
p-acceptor ligands Chemistry of lower oxidation
states very important Soft-soft interactions
very common Diamagnetic complexes
dominant Catalytic applications
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The d-block transition metals
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Main types of common ligands
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A simple classification of the most important
ligands
X
L
L2
L2X
L3
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Counting electrons
The end result will be the same
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Why is this relevant?
Stable mononuclear diamagnetic complexes generally
contain 18 or 16 electrons The reactions of
such complexes generally proceed through 18- or
16-electron intermediates
Although many exceptions can be found, these are
very useful practical rules for predicting
structural and reactivity properties
C. A. Tollman, Chem. Soc. Rev. 1972, 1, 337.
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Why 18 electrons?
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Organometallic complexes
18-e most stable
16-e stable (preferred for Rh(I), Ir(I), Pt(II),
Pd(II))
lt16-e OK but usually very reactive
gt 18-e possible but rare generally unstable
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A closer look at some important ligands
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Typical ?-donor ligands
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Other important C-donor ligands
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Other important ligands
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Other important ligands
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The M-L-X game
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Each X will increase the oxidation number of
metal by 1. Each L and X will supply 2 electrons
to the electron count.
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Now looking at compounds having a charge of 1 to
obey 18 e rule.
Elec count 4 (M) 2 (NO) 12 (L6) 18
NO is isoelectronic to CO X increases O N by 1
Elec Count 4 (M) 4 (L2) 10 (L5)
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Actors and spectators
Actor ligands are those that dissociate or
undergo a chemical transformation (where the
chemistry takes place!)
Spectator ligands remain unchanged during
chemical transformations They provide solubility,
stability, electronic and steric influence (where
ligand design is !)
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Organometallic Chemistry 1.2 Fundamental Reactions
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Fundamental reaction of organo-transition metal
complexes
Reaction D(FOS) D(CN) D(NVE)
Association-Dissociation of Lewis acids 0 1 0
Association-Dissociation of Lewis bases 0 1 2
Oxidative addition-Reductive elimination 2 2 2
Insertion-deinsertion 0 0 0
FOS Formal Oxidation State CN Coordination
Number NVE Number of valence electrons
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Association-Dissociation of Lewis acids
D(FOS) 0 D(CN) 1 D(NVE) 0
Lewis acids are electron acceptors, e.g. BF3,
AlX3, ZnX2
This shows that a metal complex may act as a
Lewis base The resulting bonds are weak and
these complexes are called adducts
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Association-Dissociation of Lewis bases
D(FOS) 0 D(CN) 1 D(NVE) 2
A Lewis base is a neutral, 2e ligand L (CO,
PR3, H2O, NH3, C2H4,) in this case the metal is
the Lewis acid
Crucial step in many ligand exchange
reactions For 18-e complexes, only dissociation
is possible For lt18-e complexes both dissociation
and association are possible but the more
unsaturated a complex is, the less it will tend
to dissociate a ligand
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Oxidative addition-reductive elimination
D(FOS) 2 D(CN) 2 D(NVE) 2
Very important in activation of hydrogen
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Oxidative addition-reductive elimination
H becomes H-
Concerted reaction
via
Ir Group 9
cis addition
CH3 has become CH3-
SN2 displacement
trans addition
Also radical mechanisms possible
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Oxidative addition-reductive elimination
Not always reversible
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Insertion-deinsertion
D(FOS) 0 D(CN) 0 D(NVE) 0
Mn Group 7
Very important in catalytic C-C bond forming
reactions (polymerization, hydroformylation)
Also known as migratory insertion for mechanistic
reasons
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Migratory Insertion
Also promoted by including bulky ligands in
initial complex
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Insertion-deinsertion The special case of
1,2-addition/?-H elimination
A key step in catalytic isomerization
hydrogenation of alkenes or in decomposition of
metal-alkyls Also an initiation step in
polymerization
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Attack on coordinated ligands
Very important in catalytic applications and
organic synthesis
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Some examples of attack on coordinated ligands
Electrophilic addition
Nucleophilic addition
Electrophilic abstraction
Nucleophilic abstraction