REACTIONS AND MECHANISMS

REACTIONS AND MECHANISMSCoordination Chemistry

TRAN BUU DANG

HOCHIMINH CITY UNIVERSITY OF EDUCATION

Gary L.Miessler, Inorg Chem 5th ed, Pearson Education, 2014, chapter 9.Housecroft, Sharpe, Inorg Chem 3rd ed, Pearson Education, 2008, chapter 20.

BackgroundTransition-state theory

a transition state without any structuresat local energy minima

an intermediate, is formed along the reaction pathway with detectable

structure.

BackgroundTransition-state theory

Large and positive equilibrium constants since ∆G < 0; both reactions are spontaneous.

The reaction canoccur quickly due to the low activation energy,

but it has a small equilibrium constant ∆G

> 0.

Substitution ReactionsInert and Labile Compounds

Substitution ReactionsKinetically Inert/Labile vs Thermodynamically Unstable/Stable Compounds

A labile complex has a very lowactivation energy for ligand substitution.

Compounds that react more slowly are called inert.

Substitution ReactionsKinetically Inert/Labile vs Thermodynamically Unstable/Stable Compounds

KR = 1030; ∆G < 0 ; in fact, the rate of reaction is too slow→ [Co(NH3)6]3+ is thermodynamically unstable, but kinetically inert.

[Ni(CN)4]2- + H2O → no any reactions

→ [Ni(CN)4]2- is thermodynamically stable, but kinetically labile.

Substitution ReactionsDissociation D Association (A)

Interchange (I)

Substitution ReactionsDissociation D

Substitution Reactions

Association (A) 51 5 1 5 2 5

1 55

1 2

5 1 2 52 5 5

1 2

[ ] [ ][ ] [ ] [ ] 0

[ ][ ][ ]

[ ] [ ][ ][ ] [ ][ ]obs

d ML XY k ML X Y k ML XY k ML XYdt

k ML X YML Xk k

d ML Y k k ML X Yr k ML XY k ML X Ydt k k

Substitution ReactionsInterchange (I)

M-Y > M-X: Ia mechanism. M-Y < M-X: Id mechanism

Substitution ReactionsPreassociation Complex

Experimental Evidence in Octahedral SubstitutionDissociation


The ligand field activation energy (LFAE), defined as the difference between the LFSE of the square-pyramidal transition state and the LFSE of the octahedral reactant


1. Oxidation state of the central ion: Central atoms with higher oxidation states have slower ligand exchange rates.

2. Ionic radius. Smaller ions have slower exchange rates.

Experimental Evidence in Octahedral SubstitutionLinear Free-Energy Relationships

CFSE (+), the rate(-) Stable bond (+), the rate(-)

Experimental Evidence in Octahedral SubstitutionThe Kinetic Chelate Effect

The ∆H associated withremoval of the first boundatom is larger than forsubsequent reattachment .

Experimental Evidence in Octahedral Substitution

The rate constants do not vary significantly with the substituting anion Y, as would be expected for a D or Id mechanism

Experimental Evidence in Octahedral Substitution

Ia and A reactions are less common with octahedral complexes due to steric hindrance.The rates depend on significantly nature of incoming ligands.

Association

Substitution Reactions of Square-Planar ComplexesSolvent-assisted substitution

rate determining step,Rate constant = kY

rate determining step, Rate constant = kS

A or Ia mechanism



The rates of reactions depend on nature of incoming L A mechanism


- The rate depends on significantly L.

- The rate of incoming L.

- The rate of leaving X.

Substitution Reactions of Square-Planar ComplexesKY path way – A or Ia - geometry preservation

Substitution Reactions of Square-Planar ComplexesKS path way – geometry preservation

Substitution Reactions of Square-Planar ComplexesThe trans effect

The trans effect: A ligand affects an other ligand placed in trans position, which trans-placed ligand is more flexible to leave for substitution reaction.


The trans effect does not always determine product.


Substitution Reactions of Square-Planar ComplexesThe explanation of trans effect - the trans influence

Sigma‐Bonding Effects

Poor trans effect: low ground state,

high transition state

σ Bonding effect: higher ground state

(trans influence)

π Bonding effect: lower transition state,

(trans effect)





The Pt‐T bond is strong, it uses a larger contribution of px,dx2-y2 orbitals of Pt atoms the Pt‐X bond is weaker, its ground state (sigma-bonding orbital) is higher in energy .

The trans influence on the basis of the relative σ donor properties of the ligands:


Pi‐Bonding Effects A strong π-acceptor Pt-T charge is removed from Ptmetal center more electrophilic and more susceptible to nucleophilic attack. formation of the 5-coordinate intermediate with a relatively strong Pt—Ybond, stabilizing the intermediate by increase in energy due to the M—X bond breaking. The energy of the transition state is lowered, reducing the activation energy.

Oxidation–Reduction ReactionsInner-sphere vs Outer-sphere reaction

Inner-sphere reaction Outer-sphere reaction

Electrons transfer through bridging ligand.

The electron exchange may occurbetween two separate coordination

spheres.

Bridge ligand must have 2 pairs of electrons at least. Ligand is unable to bridge anothers.

Oxidation–Reduction ReactionsInner-sphere reaction

Oxidation–Reduction ReactionsInner-sphere Henry-Taube reaction (Nobel 1983)

Oxidation–Reduction ReactionsInner-sphere reaction

Which is the determining step in case of a, b and c?

Oxidation–Reduction ReactionsOuter-sphere reaction



1. Sox – Sred = +/-1/2

2. For octahedral complexes, an electron

is transferred on t2g orbitals.

3. No change in geometry of reactant, product molecules.

Oxidation–Reduction Reactions

Photoredox catalysis

Cation trong X

Oxidation–Reduction ReactionsStep 1: Photoexcited state

Oxidation–Reduction ReactionsStep 2: Outersphere redox

1. Triplet state- A strong reductive agent: electrons are

ready for transferring to surroundings.- A strong oxidative agent: the unoccupied

MOs are low enough to accept electronsfrom surroundings.

2. Outer-sphere reaction (OPR): ligands are not capable of bridging.

3. A fast redox: OPR occurs faster than phosphorescence and excited states possesses a strong oxidative or reductive properties.

Oxidation–Reduction ReactionsStep 3: Relaxation to ground state

1. Oxidative quenching cycle: [Ru(bipy)3]3+ is a strong oxidative agent.

2. Reductive quenching cycle: [Ru(bipy)3]+ is a strongoxidative agent.

Oxidation–Reduction ReactionsDye sensitized solar cells

http://thep-center.org/src/article_edu_e.php?article_edu_id=47

Reactions of Coordinated LigandsHydrolysis of Esters, Amides, and Peptides

Reactions of Coordinated LigandsTemplate Reactions

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REACTIONS AND MECHANISMS