reactions of metal complexes
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8/3/2019 Reactions of Metal Complexes
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Topics
• Introduction• Molecular Structure and Bonding
• Molecular Orbital Theory
• Molecular Symmetry
• Coordination Complexes
• Reactions of Metal Complexes
• Organometallic Chemistry
Housecroft 6.11-6.13, 20.9-20.11, 25
Shriver and Atkins Chapter 7,14
Reactions of Metal Complexes
• Formation constants
– the chelate effect
– Irving William Series
– Lability
• Reaction Mechanisms
– I, A, D Mechanisms – a, d Rate Determining Step
• Substitution of Square Planar Complexes
– the trans effect
• Substitution of Octahedral Complexes
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Formation of Coordination Complexes
• typically coordination compounds are morelabile or fluxional than other molecules
• X is leaving group and Y is entering group
• One example is the competition of a ligand, L
for a coordination site with a solvent molecule
such as H2O
MX + Y MY + X
[Co(OH2)6]2+ + Cl- [Co(OH
2)5Cl]+ + H
2O
Formation Constants
• Consider formation as a series of formation
equilibria:
• Summarized as:
M + L ML]][[
][1
LM
ML K =
ML + L ML2
]][[
][ 22
LML
ML K =
M + nL MLn nn
n
nK K K K
LM
ML...
]][[
][321== β
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Values of Kn
• Typically: Kn-1>Kn
– Expected statistically, fewer coordination sitesavailable to form MLn than MLn-1
– eg sequential formation of [Al(OH2)6-x(F)x](3-x)+
Breaking the Rules
• Order is reversed when some electronic or
chemical change drives formation
– jump from a high spin to low spin complex
• Fe(bipy)2(OH2)2 t 2g 4eg 2 high spin• Fe(bipy)3 t 2g
6 low spin
N N
2,2'-bipyridine = bipy
Fe(bipy)2(OH
2)22+
+ bipy Fe(bipy)32+
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Chelate Effect
• Compare: K1 to β2 for:
• Basically equivalent chemistry but for Cu2+
log K1 =10.6 log β2 =7.7
• chelated complex is three orders of magnitude morestable
• chelate effect : the enhanced stability of a chelatedcomplex over its non-chelating analog
• attributed to the change in entropy, chelation tradestwo restricted solvent molecules for one bound ligand
M(OH2)2
2+
+ en M(en)2+
+ 2H2O
M(OH2)22+ + 2NH3 M(NH3)2
2+ + 2H2O
Ring Formation and Electron Delocalization
• Ability to form rings with metal center
improves stability
– particularly five or six membered rings
• Additionally, ligands with aromatic rings can
behave as pi acceptors and form backbonding complexes
Ru
NN
N
N
NN
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Irving William Series
• Values of log Kf for 2+ions including transition
metal species
• Kf series for transition
metals:
Mn2+< Fe2+< Co2+< Ni2+< Cu2+>Zn2+
Irving Williams Series
• Partially explained by electrostatics: smaller
metal centre, same charge = greater charge
density
• Based on electrostatics we expect stabilities
which vary as:
Mn2+
< Fe2+
< Co2+
< Ni2+
> Cu2+
>Zn2+
• Irving William Series gives Cu2+ more stable
than Ni2+
– Because of Jahn Teller Distortion
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Ni2+ vs Cu2+ Kf
• Stepwise Kf for displacement of H2O by
NH3 ligands from aquated Ni2+ and Cu2+
Reaction Mechanisms of d Metal Complexes
• We’ve been considering the equilibrium
formation
• Rate is important for understanding
coordination complex chemistry
– Inert: species that are unstable but survive for
minutes or more – Labile: species that react more rapidly than inert
complexes
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Labile vs. Inert
• General Rules: – For 2+ ion, d metals are moderately labile
particularly d 10 (Hg2+, Zn2+)
– Strong field d 3 and d 6 octahedral complexes are
inert .i.e. Cr(III) and Co(III)
– Increasing Ligand Field Stabilization Energy
improves inertness
– 2nd and 3rd row metals are generally more inert
Ligand Field Stabilization Energy (LFSE)
• Consider the energy of the d orbitals before crystal
field splitting relative to the first three possible
electronic configurations
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LFSE for Oh Geometry
0t2g6eg
4d10
-0.6t2g6eg
3d9
-1.2t2g6eg
2d8
-1.8t2g
6eg
1-0.8t2g
5eg
2d7
-2.4t2g6eg
0-0.4t2g4eg
2d6
-2.0t2g5eg
00t2g3eg
2d5
-1.6t2g4eg
0-0.6t2g3eg
1d4
-1.2t2g3eg
0d3
-0.8t2g2eg
0d2
-0.4t2g1eg
0d1
FSE(∆o)configFSE (∆o)config
Low SpinHigh Spindn
LFSE: e- configuration determines stabilization
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Associative vs Dissociative Reactions
• Ligand substitution reactions are either
associative or dissociative – Associative: reaction intermediate has higher
coordination number than reactants or products
• lower coordination number complexes
• Rates depend on the entering group
– Dissociative: reaction intermediate has lower
coordination number than reactants or products
• Octahedral complexes and smaller metal
centers• Rates depend on leaving group
Patterns of Reactivity
• Formation constants tell us about
thermodynamics
• Kinetics requires a different measure:
nucleophilicity
– Ligand displacement are nucleophilic substitution
reactions – The rate of attack on a complex by a given ligand
(Lewis Base) relative to the rate of attack by a
reference base.
• Rates span from 1 ms to 108 s
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Ligand Labels for Nucleophilic Substitutions
• Three types of ligands can be important: – Entering Ligand: Y
– Leaving Ligand: X
– Spectator Ligand
• Species that neither enters nor leaves
• Particularly important when located in a Trans
position, designated T
Reaction Mechanisms
• Associative - A (2 steps)
• Dissociative - D (2 steps)
• Interchange (1 continuous process)
MLnX + Y ML
nXY ML
nY + X
MLnX + Y MLn + X + Y MLnY + X
MLnX + Y Y--ML
n--X ML
nY + X
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Rate Determining Step
• also denoted associative or dissociative• associative (lowercase a)
– the rate depends heavily on the entering group
• dissociative (lowercase d)
– the rate is independent of the entering group
[PtCl(dien)]+ + I- [PtI(dien)]+ + Cl-
[PtCl(dien)]+ + Br - [PtBr(dien)]+ + Cl-
[Ni(OH2)6]2+ + NH
3[Ni(OH
2)5(NH
3)]2+ + H
2O
Substitution of Square Planar Complexes
• substitution of square planar complexes is
almost always Aa mechanisms
– rate depends on the entering group
– rate determining step is the M-Y bond formation
• impacted by the Trans effect
– the ligand trans to the leaving ligand (X) can alter the reaction rate
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Square Planar Substitution: The Trans Effect
• when the ligand, T, trans to the leaving groupin square planar complexes effects the rate of substitution
• If T is a strong σ donor or π acceptor, the rateof substitution is dramatically increased
• why? – if T contributes a lot of e- density (is a good σ
donor) the metal has less ability to accept electrondensity from X (the leaving ligand)
– if T is a good π acceptor , e- density on the metal isdecreased and nucleophilic attack by Y isencouraged
Trans Effect Strengths
• Trans effect is more pronounced for σ donor
as follows:
OH-<NH3<Cl-<Br -<CN-,CO, CH3-<I-<PR3
• Trans effect is more pronounced for a π
acceptor as follows:
Br -<Cl-<NCS-<NO2-<CN-<CO
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Using the Trans Effect
• Suggest a means to synthesize cis and trans[PtCl2(NH3)2] from [Pt(NH3)4]
2+ and [PtCl4]2-
Square Planar Substitution: Steric Effects
• steric crowding reduces the rate of A
mechanisms and increases D mechanisms
• simply a spatial phenomenon:
– less room around the metal means that a higher
coordination number transition state is higher
energy• eg cis-[PtXL(PEt3)2]
• rate varies with L
• pyridine > 2-methyl py >
2,6-dimetyl py
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Square Planar Substitution: Stereochemistry
• observing the final product stereochemistry
can provide information on the mechanismand intermediate lifetimes
Square Planar Substitution: Volume of Activation
• changes in volume along a reaction pathway
can be determined
• usually by observing reaction rate as a
function of pressure
• a negative ∆V‡ suggests an associative
complex
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Square Planar Substitution: Entropy of
Activation
• the change in entropy from the reactants tothe activated complex is ∆S‡
• determined by the temperature dependence
of the rate
• associative mechanism has –’ve ∆S‡
• as expected from increasing order of the
system by loss of freedom for the entering
group without release of the leaving group
Substitution of Square Planar Complexes
• Trans Effect – ligand trans to X can increase
substitution if it is a good σ donor or π acceptor
• Steric Effects – bulky cis ligands reduce Y
nucleophilic attack
• Stereochemistry – cis/trans conserved for A
mechanism unless activated complex is longlived
• ∆V‡ and ∆S‡ are both negative for A mechanism
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Substitution of Octahedral Complexes
• I is the most important reaction mechanismfor substitution of Oh complexes
• but is it Ia or Id – recall it depends on the rate determining step
being Y—M formation vs M—X breaking
– associative (lowercase a)
• the rate depends heavily on the entering group
– dissociative (lowercase d)
• the rate is independent of the entering group
Eigen-Wilkins Mechanism
• The standard mechanism for Oh I
substitutions reactions
• Based on the formation of an “encounter
complex”
• Fast pre-equilibrium:
• Followed by product formation:
ML6
+ Y {ML6,Y}
]][[}],[{
6
6
Y ML
Y ML K E =
{ML6,Y} product }],[{ 6 Y MLk rate =
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Eigen-Wilkins Mechanism II
• The rate expression can be written in terms of the KE so that:
• Where [C]tot is the total of all of the complex
species
• If KE[Y] << 1 then the rate becomes:
][1
][][
Y K
Y C kK rate
E
tot E
+=
][][ Y C k ratetot obs
=
Using Eigen Wilkins
• kobs = k KE so we can get k
• Now test k to see if it varies with Y or not so
we can assign Ia or Id
• Whew!
• See table 14.6 for experimental data
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Oh Substitution General Rules
• Most 3d metals undergo Id substitutions – I.e. the rate determining step is independent of the
entering group and primarily is the breaking of the
M—X bond
• Larger metals (4d, 5d ) lean towards Ia
• Also low d electron density encourages partly
Ia characteristics
Oh: Effects of Ligands
• Leaving Group
– Nature of X is important as expected for Id as bond
breaking of M-X is the rate determining step
• Spectator ligands (cis-trans effect)
– No clear trans effect for Oh complexes – In general, good spectator sigma donors will
stabilize the complex after the departure of the
leaving group
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Oh: Steric Effects on Substitution
• steric crowding around the metal centrefavors dissociative activation
• Dissociative activation relieves crowding
around the complex
• Steric crowding has been qualitatively and
quantitatively explored
– Tolman Cone Angle
– See Table 14.7
Octahedal Substitution and ∆V‡
• For I mechanism,
∆V‡ is not large but
Ia tends to be –’ve,
Id tends to be +’ve• decreasing d
number shows
tendancy towards Iamechanism Ni2+
Co2+
Fe2+
Mn2+Cr
2+
V2+
Ti2+
M2+
+7.28
+6.17
+3.86
-5.454
-4.13
2
∆V‡d elec.
M(OH2)6
+ H217O M(OH
2)5(17OH
2) + H
2O
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2
Oh Stereochemistry of Substitution
• More complicated than for Td complexes• Example: cis- or trans- [CoAX(en)2]
2+
• cis complexes tends to retain cis
• trans complexes can isomerize depending on
the spectator ligand, depends on geometry of
the activated complex
– Trigonal bipyramidal results in isomerization
depending on where Y enters
– Square planar leads to retention of
stereochemistry
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2
Isomerization Reactions
• Similar to substitution reactions• Berry Pseudorotation mixes axial and equatorial
positions in a 5 coord TBP species
• Both square planar complexes which undergo A
mechanisms or Oh complexes which undergo D or Id
mechanisms involve a 5 coordinate state so …
isomerization is possible
Twisted Oh Isomerizations
• Oh complexes may also isomerize via “twist”
mechanisms
• Does not require loss of ligands or breaking
bonds, just depends on energy barriers
between confirmations
– Bailar Twist (a) – Ray Dutt Twist (b)
• Both occur via trigonal prismatic
confirmation
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2
Twists
Redox Reactions
• Requires transfer of electrons in form of
straight electrons
– Like electrochemical cell, transfer from one metal
to another
– Transfer of group of ligands along with their
electrons to effectively reduce or oxidize a metal
centre
– Shriver and Atkins: Chapter 14
– Housecroft and Sharpe: Chapter 25
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2
Redox Reactions
• Two reaction mechanisms – Inner sphere
• Requires formation of bridged bimetallic
species
• results in ligand transfer at the same time
– Outer sphere
• No bridging ligand involved
• Direct transfer of electrons between the metal
centres
Outer Sphere Reaction Mechanisms
• Readily identified when no ligand transfer occurs between the species
• Easier to identify when complexes are inertwith respect to ligand substitution
• Born Oppenheimer Approximation – Electrons move faster than nuclei
– Complexes reorganization can be considered in aseparate step from electron transfer
• Marcus Equation – Electron transfer requires vibrational excited
states, shape of potential energy well determinesrate of transfer
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2
Inner Sphere Reactions
• Require the presence of bridging ligands – Ligands with multiple pairs of electrons to donate
• Rate of electron transfer is dependent on the
ligands that are present
• See table 14.11 in Shriver and Atkins or table
25.8 in Housecroft and Sharpe
Cl-S C N- N N C N-
Inner Sphere Reaction Steps
• Formation of Bridged Complex
• Electron Transfer
• Decomposition into Final Products
MIIL6 + XMIIIL5' L5MII XMIIIL5
' + L
L5M
II
XM
III
L5
' L5
MIII XMIIL5
'
L5MIII XMIIL5' products L5MIII
X MIIL5'+
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Rate Determining Step
• Usually the electron transfer step• However formation of bridging complex or the
decomposition could also limit the rate
• Where rds is electron transfer
– Good conjugation could provide a simple path for
the electron
• Studied via construction of bridging ligand
systems as models
Conclusions
• Reaction mechanisms
– A basic description of different mechanisms for
• Ligand exchange
• Isomerization
• Electron transfer
– Emphasis on ligand substitution reactions
• Determination of I, A, D mechanisms
• a vs d activation
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