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Acid hydrolysis or aquation reactions may be defined as the reactions in which an aquo complex is formed due to the replacement of a ligand by water molecule It 

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Coordination Chemistry IV: Reactions and Mechanisms 12

Rule of

play ??

Background

Studying chemical kinetics Understanding reaction mechanism Finding ways of facilitating the reaction

tC tA

BAkRate

nm

ĺ 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.

NO 2 (g)+CO(g) ĺ 2 (g)

Experimentally,

2 2 ][NOkRate

Possible mechanism

NO 3 (g)+CO(g) ĺ 2 (g)+CO 2 (g) NO 2 (g)+NO 2 (g) ĺ 3 (g)+NO(g) 2 213
][][NOktNORate rate-determining step NO 2 (g)+CO(g) ĺ 2 (g) 2 212
][][][NOktCO tNORate

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. transition state intermediate

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 tA

BAkRate

nm 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

Background

Reaction types of transition metal complexes

• Substitution reactions • Dissociation reactions • Addition reactions • Oxidation-reduction reactions • Reactions of coordinated ligands B AC D X E B AC D Y E +Y + X B AC D X E B AC D E -X + X B AC D E B AC D Y E +Y M 2+ B AC D X E M 3+ B AC D Y E M 3+ B ACH 3 D X E M 2+ B AC D Y E

Substitution Reactions Inert and Labile Compounds

Examples of substitution reactions

[Ni(H 2 O) 6 2+ + 6 NH 3 [Ni(NH 3 6 2+ + 6 H 2 O green blue

Successive addition of HNO

3 , NaCl, H 3 PO 4 , KSCN, NaF to aq. soln of Fe(NO 3 3 •9H 2 O

These reactions are fast. Complexes, which react

rapidly, are called labile.

Taube's criterion - t

1/2

Labile

Thermodynamically unstable, but kinetically inert

[Fe(H 2 O) 5 (F)] 2+ thermodynamically stable, but kinetically labile Be careful! : thermodynamically ____, kinetically ______

Inert compound

is easier to be studied.

Substitution Reactions Inert and Labile Compounds

These reactions are fast. Complexes, which react

rapidly, are called labile.

Taube's criterion - t

1/2 [Fe(H 2 O) 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

Substitution Reactions Mechanisms of Substitution

Dissociative (D) Mechanism ML

n

ĺ ML

n

ĺ ML

n Y + 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 S N 1).

Associative (A) Mechanism ML

n

X + Y ĺ ML

n

XY ĺ ML

n Y + X The incoming ligand adds to the complex, and an intermediate with a increased coordination number is formed. (analogous to S N 2).

Interchange (I) Mechanism ML

n

X + Y ĺ ML

n Y + X

Y---ML

n X The incoming ligand is presumed to assist in the reaction, but no detectable intermediates appear. dissociative interchange (I d ) and associative interchange (I a

Y---ML

n ---X Substitution Reactions Kinetic Aspects of Reaction Pathways

Dissociative (D) Mechanism ML

n

ĺ ML

n

ĺ ML

n Y + 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 S N 1). dtd]ML[ 5

Mechanism

Steady-state approximation

]ML[ 5

Rate law

[Y][X]X][Y][ML][Y][MLY][ML 215
12 525
kkkkkdtd Substitution Reactions Kinetic Aspects of Reaction Pathways

Associative (A) Mechanism ML

n

X + Y ĺ ML

n

XY ĺ ML

n Y + X The incoming ligand adds to the complex, and an intermediate with a increasedquotesdbs_dbs17.pdfusesText_23