Dissociative substitution
Encyclopedia
Dissociative substitution describes a pathway by which compounds interchange ligand
s. The term is typically applied to coordination and organometallic
complexes, but resembles the Sn1 mechanism
in organic chemistry
. The opposite pathway is associative substitution
, being analogous to Sn2 pathway
. Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called interchange mechanisms.
Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile
has no influence on this rate. An intermediate of reduced coordination number can be detected. Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry
. The entropy of activation
is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate determining step.
effects, and others. Many metal mono- and dications form octahedral aquo complexes. Most of these exchange processes occur via an interchange pathway that has more or less dissociative character. Rates vary by a factor of 1018, [Ir(H2O)6]3+ being the slowest and [Na(H2O)6]+ being one of the fastest for octahedral complexes. Charge has a significant influence on these rates but non-electrostatic effects are also important.
of cobalt(III) ammine (NH3-containing) halide complexes are deceptive, appearing to be associative but proceeding by a pathway that is dissociative in character. The hydrolysis of [Co(NH3)5Cl]2+ follows second order kinetics: the rate increases linearly with concentration of hydroxide as well as the starting complex. Studies show, however, that in the hydroxide deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+. In this monocation, the chloride spontaneously dissociates from this conjugate base of the starting complex. This pathway is called the Sn1CB mechanism
.
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. The term is typically applied to coordination and organometallic
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
complexes, but resembles the Sn1 mechanism
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
. The opposite pathway is associative substitution
Associative substitution
Associative substitution describes a pathway by which compounds interchange ligands. The terminology is typically applied to coordination and organometallic complexes, but resembles the Sn2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to Sn1...
, being analogous to Sn2 pathway
SN2 reaction
The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophilic center and bonds to it, expelling another group called a leaving group. Thus the incoming group replaces the leaving group in one step...
. Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called interchange mechanisms.
Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
has no influence on this rate. An intermediate of reduced coordination number can be detected. Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
. The entropy of activation
Entropy of activation
The entropy of activation is one of the two parameters typically obtained from the temperature dependence of a reaction rate, when these data are analysed using the Eyring equation. Symbolized ΔS‡, the entropy of activation provides clues about the molecularity of the rate determining step in a...
is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate determining step.
Dissociative interchange pathway
Interchange pathways apply to substitution reactions where intermediates are not observed, which is more common than pure dissociative pathways. If the reaction rate is insensitive to the nature of the attacking nucleophile, the process is called dissociative interchange, abbreviated Id. An illustrative process comes from the "anation" (reaction with an anion) of cobalt(III) complexes:-
- [Co(NH3)5(H2O)]3+ + SCN-ThiocyanateThiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
{[Co(NH3)5(H2O)], NCS}2+ - {[Co(NH3)5(H2O)], NCS}2+
[Co(NH3)5NCS]2+ + H2O
- [Co(NH3)5(H2O)]3+ + SCN-
Water exchange
The exchange between bulk and coordinated water is of fundamental interest as a measure of the intrinsic kinetic lability of metal ions. This rate is relevant to toxicity, catalysis, magnetic resonance imagingMagnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
effects, and others. Many metal mono- and dications form octahedral aquo complexes. Most of these exchange processes occur via an interchange pathway that has more or less dissociative character. Rates vary by a factor of 1018, [Ir(H2O)6]3+ being the slowest and [Na(H2O)6]+ being one of the fastest for octahedral complexes. Charge has a significant influence on these rates but non-electrostatic effects are also important.
Sn1CB mechanism
The rate for the hydrolysisHydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of cobalt(III) ammine (NH3-containing) halide complexes are deceptive, appearing to be associative but proceeding by a pathway that is dissociative in character. The hydrolysis of [Co(NH3)5Cl]2+ follows second order kinetics: the rate increases linearly with concentration of hydroxide as well as the starting complex. Studies show, however, that in the hydroxide deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e., [Co(NH3)4(NH2)Cl]+. In this monocation, the chloride spontaneously dissociates from this conjugate base of the starting complex. This pathway is called the Sn1CB mechanism
Sn1CB mechanism
The SN1CB mechanism describes the pathway by which many metal amine complexes undergo substitution, that is ligand exchange. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base to give the corresponding polyamine metal hydroxide:The rate law for the reaction...
.