reactions of metal complexes

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



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



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+



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



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



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



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



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



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



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



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



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



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



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



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



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 =



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



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



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



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



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



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



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



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'+



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

reactions of metal complexes

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