Transcript Complexes Ligand-Substitution Reactions

Mechanism for Complexes
Ligand-Substitution Reactions
by
M. MAHFUDZ FAUZI S., S.Pd., M.Sc.
STUDY PROGRAM OF CHEMICAL EDUCATION
DEPARTMENT OF MATHEMATICS AND NATURAL SCIENCES
FACULTY OF TEACHER TRAINING AND PEDAGOGY
UNIVERSITY OF LAMPUNG
References:
1. Douglas, B., McDaniel, D.H., and Alexander, J.J. 1983.
Concepts and Models of Inorganic Chemistry.
Singapore. John Wiley & Sons, Inc.
2. Basolo, F. and Pearson, R.G. 1973. Mechanisms of
Inorganic Reactions. New Delhi. Wiley Eastern Private Ltd.
3. Miessler, G.L. and Tarr, D.A. 2003. Inorganic Chemistry.
London. Prentice-Hall International.
4. Bakac, A. 2010. Physical Inorganic Chemistry.
Reactions, Processes, and Applications. Canada. John
Wiley & Sons, Inc.
5. Shriver, D.F. and Atkins, P.W. 2010. Inorganic Chemistry.
Oxford. Oxford University Press.
6. Sugiyarto, K.H. 2009. Dasar-dasar Kimia Anorganik
Transisi. Yogyakarta. Jurdik Kimia FMIPA UNY.
Complexes Ligand-Substitution Reactions
K3[Fe(CN)6]
+
the 2nd coordination sphere
the 1st coordination sphere
3K+
On ML6 complex
1. Same ligand, have same
probability for exchange.
2. Different ligand, only
equatorial position that
have the bigger
probability for exchange.
La
Lb
Le
M
Lc
Ld
Lf
Chemical reactions in which
the composition of the first
coordination sphere around
a metal change are called
ligand substitution reactions.
Mechanisms for Ligand Substitution Reactions
Gray and Langford (1968) have labeled mechanisms for
ligand substitution reactions as the intimate and
stoichiometric mecanisms, respectively:
1. Dissociative, intermediate detectable SN1
2. Associative, intermediate detectable SN2
3. Interchange, intermediate undetectable
Experimental Test of Mechanism
Two frequently studied reactions are:
1. Aquation (sometimes called acid hydrolisis)
L5MX + H2O → L5M(H2O) + X
Co3+
Co3+
+ H2O
Entering Ligand
Leaving Ligand
Non Leaving Ligand
+ Cl-
2. Anation
L5M(H2O) + Y → L5MY + H2O
Co3+
Co3+
+ Cl-
Entering Ligand
Leaving Ligand
Non Leaving Ligand
+ H2O
Thermodynamically, complexes devided into two groups:
1. Stabile complexes
2. Unstabile complexes
Kinetically, Taube (1952) have devided complexes into two
groups:
1. Complexes having t1/2 30 sec for substitution labile.
2. Complexes with longer t1/2 inert.
Gray and Langford (1968) have devided metal ions into four
classes, based on exchage rate:
1. Class I, very fast exchange of water occurs, k ≥ 108 sec-1.
The alkali and alkali earth metal ions (except for Be(II)
and Mg(II)).
2. Class II, exchange rate constants are between 104 and
108 sec-1. The divalent first row transition metal ions
(except for V(II), Cr(II), and Cu(II)).
3. Class III, exchange rate constants are between 1 and
104 sec-1. Be(II), V(II), Al(III), Ga(III) metal ions and several
the trivalent first row transition metal ions.
4. Class IV, exchange rate constants fall between 10-6 and
10-3 sec-1. Metal ions are inert in Taube’s sense (Cr(III),
Co(III), Rh(III), Ir(III), and Pt(II)).
Dissociative Mechanisms
Associative Mechanisms
Steric and Electronic Effects of Inert Ligands
Steric Effects
Crowding arround the metal ion would be expected to
retard the rates of reactions or to speed the rates of
reactions.
Would be expected to retard the rate of reactions that
occur by an a mechanism.
Would be expected to speed the rate of reactions that
occuring via a d mechanism.
Electronic Effects
The steric requirements of these ligands do not differ much
and the rate increase is considered to arise from enhanced
possibilities for electron delocalization into unsaturated
ligand, making the metal center softer, and stabilizing the
transition rate.
If the cis ligand is a good π-donor, it can supply electrons to
the electron-deficient Co, thereby stabilizing the transition
state and lowering activation energy. As a consequence,
cis complexes where L is good π-donor react more rapidly.
When L is trans to leaving group X, no π-donation into the
vacant Co orbital can occur without rearrangement to a
trigonal bipyramid. The energy of rearrangement cause the
activation energy to be larger and the rate lower than
those of corresponding cis complexes.
Base Hydrolisis
Dissociative Mechanism
Stereochemistry of Oh Substitution Reactions
A
Associative Mechanism
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