|Table of contents
|Reaction Mechanism of Transition Metal Complexes – I:
|Inert and Labile Complexes
|Mechanisms for Ligand Replacement Reactions
|Formation of Complexes from Aquo Ions
|Ligand Displacement Reactions in Octahedral Complexes- Acid Hydrolysis, Base Hydrolysis
|Racemization of Tris Chelate Complexes
|Electrophilic Attack on Ligands
The metal complexes in which the rate of ligand displacement reactions is very fast and hence show high reactivity are called as labile Complexes and this property is termed as lability. On the other hand, the metal complexes in which the rate of ligand displacement reactions is very slow and hence show less reactivity are called as inert complexes and this property is termed as inertness.
Thermodynamic stability is the measure of the extent to which the complex will form or will be transformed into another complex when the system has reached equilibrium while kinetic stability refers to the speeds at which these transformations take place. The thermodynamic stability depends upon the energy difference of the reactant and product if the product has less energy than that of the reactant, it will be more stable than the reactant. The thermodynamic stability of metal complexes is calculated by the overall formation constant. If the value of log β is more than 8, the complex is considered as thermodynamically stable.
The kinetic stability of the complex depends upon the activation energy of the reaction. If the activation energy barrier is low, the reaction will take place at a higher speed. These types of complexes are also called as kinetically unstable. If the activation energy barrier is high, the substance will react slowly and will be called as kinetically stabilized or inert. There is no correlation between thermodynamic and kinetic stability. Thermodynamically stable products may labile or inert and the vice versa is also true.
According to the valence bond theory of chemical bonding, octahedral metal-complexes can be divided into two types.
Octahedral complexes react either by SN1 or SN2 mechanism in which the intermediates are five and seven-coordinated species, respectively. In both cases, the symmetry of the complex is lowered down and due to this change in crystal field symmetry, the crystal field stabilization (CFSE) value also changes. The cases for lability and inertness are:
Hence, the gain of crystal field stabilization energy will make complex labile while the loss of CFSE will make complex inert. The calculation of CFSE relies upon the following assumptions:
1. All six-coordinated complexes should be treated as perfect octahedral even if the mixed ligands are present.
2. The inter-electronic repulsive forces arising from d-subshell can simply be neglected.
3. The Dq-magnitude for reacting as well as the intermediate complexes are assumed to be the same though they might have considerably different values.
4. The Jahn-Teller distortion should also be neglected in all calculations.
➤ Evidence for the lability and inertness: The ligand displacement can be dissociative or associative depending upon the nature of the reaction.
(i) For SN1 or dissociative pathway, the 5-coordinate intermediate can be trigonal-bipyramidal or squarepyramidal. However, it has been observed that the dissociative mechanism occurs through a square-pyramidal intermediate. Hence, the gain or loss of the crystal field stabilization energy can be calculated as:
CFSE gain or losss = CFSE of Squarepyramidal Intermediate – CFSE of Octahedral Reactant
If the CFSE gain-or-loss is negative, it means that activation energy is zero because it cannot be negative.
Table 1. CFSE values of high-spin (HS) and low-spin (LS) octahedral complexes undergoing ligand displacement reactions through SN1 mechanism
From Table 1, we can say that:
Metal complexes with d0, d1, d2, d10 are labile in nature and undergo fast ligand displacement through the dissociative pathway. High spin metal complexes with d4, d5, d6, d7 are also labile in nature and react quickly through the dissociative pathway. Low spin complexes of d7 metal ions are also found to be labile due to CFSE gain.
On the other side, d3 and d8 metal complexes are inert in nature and undergo slow ligand displacement through the dissociative pathway. Moreover, low spin complexes with d4, d5 and d6 metal complexes are also inert due to loss of CFSE during the SN1 mechanism. Using the order of CFSE loss, the reactivity can be represented as d5 > d4 > d8 > d3 > d6.
(i) For SN2 or associative pathway, the 7-coordinate intermediate can be a pentagonal-bipyramidal or octahedral wedge. However, it has been observed that the associative mechanism preferably occurs through an octahedral-wedge intermediate. Hence, the gain or loss of the CFSE can be calculated as:
CFSE gain or loss = CFSE of Octahedral-Wedge Intermediate – CFSE of Octahedral Reactant
Table 2. CFSE values of high-spin (HS) and low-spin (LS) octahedral complexes undergoing ligand displacement reactions through SN2 mechanism
The following conclusions can be drawn from the data listed in Table 2.
Metal complexes with d0, d1, d2, d10 are labile in nature and undergo fast ligand displacement through the associative pathway. High spin metal complexes with d4, d5, d6, d7 are also labile in nature and react quickly through the associative pathway. Low spin complexes of d7 metal ions are also found to be labile due to CFSE gain. It can be seen that d4 low spin are also labile in nature.
On the other side, d3 and d8 metal complexes are inert in nature and undergo slow ligand displacement through the associative pathway. Moreover, low spin complexes with d5 and d6 metal complexes are also inert due to the loss of CFSE during the SN1 mechanism. Using the order of CFSE loss, the reactivity can be represented as d5 > d8 > d3 > d6.
The kinetic stability of non-transition metal complexes can be rationalized from the valence bond theory (VBT) as well as from the perspectives of crystal field theory (CFT). According to the valence bond model, all of the non-transition metal complexes are outer-orbital in nature; and therefore, are expected to show labile behavior. Similarly, the predictions of kinetic stability of octahedral complexes of non-transition metals are also labile because whatever the path is followed, associative or dissociative, the loss of CSFE will always zero. Nevertheless, the overall trend kinetic stability of transition metal complexes depends upon a number of factors discussed below.
Now although the lability of transition metal complexes mainly depends upon the gain or loss of CFSE during the formation intermediate (what we have already discussed in this section previously), the geometry also plays some role in the same. In other words, in addition to the lability of non-transition metal complexes, ‘Figure 4’ may also be used to explain some lability profiles in transition metal complexes. For instance, the rate of exchange of CN− by 14CN− in [Ni(CN)6]2− is greater than what is observed for [Mn(CN)6]3− and [Co(CN)6]3− complexes. This can be attributed to the easy formation of an activated complex with the incoming ligand, which in turn, facilitates the removal of the previously attached ligand.
The ligand displacement in metal complexes is said to have been taken place if one of the previously attached ligands got replaced by another ligand from its coordination sphere. The scheme can be shown as:
MAnL + E → MAnE + L ...(2)
Where ligand L is the leaving group present in the complex, E is the entering ligand which is nucleophilic in nature. The coordination number of the complex remains the same.
In octahedral complexes, the replacement of the ligand can occur through dissociative, associative or by interchange mechanism. It has also been observed that most of the ligand displacement takes place through the interchange rout rather than purely associative or dissociative.
In square-planar complexes, the ligand displacement is much more favorable through the associative route than that of dissociative which can be understood in terms of low steric crowding due to lesser coordination number. The general ligand displacement can be written as:
The intermediate state is trigonal-bipyramidal and undergoes rapid Berry-pseudo-rotation followed by the elimination of the leaving group.
Figure 10. The general reaction mechanism for ligand displacement reactions in square-planar complexes.
Figure 11. The typical reaction coordinate diagram for ligand displacement reactions in square-planar complexes through the associative mechanism.
A more in-depth visualization of ligand displacement reactions in square-planar complexes is given below in which the involvement of Berry-pseudorotation is depicted more precisely.
Figure 12. Involvement of Berry-pseudorotation in the ligand displacement in square-planar complexes.
The complex formation from the aquo ions yields the assembly containing metal ions with only water as ligands. These complexes are the major components in aqueous solutions of many metal salts, like metal sulphates, perchlorates and nitrates. The formula for metal-aquo complexes [M(H2O)n]z+ where the value of z generally varies from +2 to +4. The metal-aquo complexes play some very important roles in biological, industrial and environmental aspects of chemistry. Now though the homoleptic aquo complexes (only with H2O as the ligands attached) are very common, there are many other complexes that are known to have a mix of aquo and other ligand types.
The most common stereochemistry for metal-aquo complexes is octahedral with the formula [M(H2O)6]2+ and [M(H2O)6]3+; nevertheless, some square-planar and tetrahedral complexes with the formula [M(H2O)4]2+ are also known. A general discussion on the different types and other properties is given below.
Most of the transition metal elements from the first transition series and some alkaline earth metals form hexa-coordinated complexes when their corresponding salts are dissolved in water. Some of the most studied hexa-coordinated complexes are given below.
The metal-aquo complexes that exist with coordination numbers lower than six are very uncommon but not absent from the domain. For instance, Pd2+ and Pt2+ form [M(H2O)4]2+ complexes with the squareplanar stoichiometry; and a rare tetrahedral aquo complex [Ag(H2O)4]+ is also known. Some of the most studied hexa-coordinated complexes are given below.
The metal-aquo complexes of the trivalent lanthanides are eight- and nine-coordinated, which is obviously due to the large size of the metal ions. In past, the coordination number of Ln3+ ions in their aquo complexes was somewhat more or less controversial; however, nowadays, advanced characterization techniques like O17 NMR or density functional studies number of coordinated water molecules decreases from nine to eight with the decrease of the ionic radius i.e La3+ to Lu3+. Some of the most studied eight and ninecoordinated complexes are given below.
The [Ln(H2O)9]3+ ions have a trigonal triprismatic geometry with a slightly distorted D3 symmetry, while the [Ln(H2O)8]3+ ions possess a square antiprismatic geometry with a slightly distorted S8 symmetry.
There are some metal-aquo complexes which do possess metal-metal bonds. Two of the most studied examples are [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+ which have eclipsed and staggered conformations, respectively. It should also be noted that metal-metal in [Mo2(H2O)8]4+ is of quadruple nature (four bond order).
The main reactions shown by metal-aquo complexes are the electron-transfer, ligand exchange, and acid-base reactions of the O-H bonds. A general discussion on all three is given below.
The general scheme for the ligand displacement reactions in octahedral complexes can be shown as:
MA5L + E → MA5E + L ...(12)
Where ligand L is the leaving group present in the complex, E is the entering ligand which is nucleophilic in nature. The coordination number of the complex remains the same. Moreover, if the entering group E is H2O or OH– in aqueous solution, the study of ligand displacement become more important due to extremely wide application domain. Some of the most prominent reactions in ligand substitution in six-coordinated complexes are discussed in detail.
The tris chelate metal complexes exist in two enantiomeric forms, called as Λ and Δ configurations.
The point group symmetry for the above system D3. There are also geometrical isomers when the bidentate ligands are unsymmetrical in nature like glycinato. The total number isomer, in that case, is four as:
The interconversion or the racemization of tris chelate complexes in case symmetrical ligands can take place via with or without the rupturing of the metal-ligand bond.
There is a special class of ligand substitution reactions in which ligand displacement occurs without the breaking of metal-ligand bonds. Experimental studies suggested that these reactions take place via an electrophilic attack on the ligand already attached to the metal center. Consider the preparation of [Co(NH3)5(H2O)]3+ complex from [(NH3)5–Co–CO3]+ cationic species:
It has been observed that the no isotopically labeled oxygen is found in the aquo complex which indicates that no metal-ligand bond-breaking has actually been taken place during the course of the whole substitution process.
Mechanism: The whole process can be summed up into the following steps
(1) Electrophilic or the proton attack on the oxygen atom of the ligand bonded to the trivalent cobalt.
(2) Expulsion of CO2 and H2O to form hydroxo complex.
(3) Protonation of the hydroxo complex.
It is also worthy to mention that the reaction described above is a decarboxylation rather than the acid hydrolysis.
The similar behavior can be observed in the reaction of NO2− group with pentaammineaquocobalt(III) ion. When the studies involving isotopic labelling were carried out, we came to know that show that the oxygen of bound water is actually the same to what is used by NO2− to bind with metal centre. This unusual result can be explained by the following mechanism.
Hence, in both of the reactions given above, the displacement of previously attached ligand octahedral complexes occurs without metal-ligand bond breaking.
Q 1. How does the ligand field affect the choice between SN1 and SN2 paths in the substitution reactions of octahedral complexes? Also, explain how is this choice influenced by the basicity and π bonding capacity of a non-reacting ligand?
Q 2. Discuss the mechanism of acid hydrolysis taking the example of the octahedral complex of Co(III).
Q.3 What do you understand by SN1CB? Explain with example.
Q 4. Explain the stereochemistry of SN2 substitution reactions of octahedral complexes.
Q 5. What is base hydrolysis? Discuss the possible mechanisms.
Q 6. How does the racemization of tris chelate complexes take place?
Q 7. Explain the mechanism of nucleophilic substitution reactions in octahedral complexes.
Q 8. What are metal-aquo complexes? Also, draw and discuss the structure of eight and nine coordinated aquo complexes of trivalent lanthanide ions.
Q 9. Give a brief discussion on important reactions of metal-aquo complexes.
|1. How do transition metal complexes with inert and labile properties differ in terms of their reaction mechanisms?
|2. What are the mechanisms involved in ligand replacement reactions in transition metal complexes?
|3. How do aquo ions form complexes and what is the mechanism behind it?
|4. What are the mechanisms involved in ligand displacement reactions in octahedral complexes, specifically acid and base hydrolysis?
|5. How do tris chelate complexes undergo racemization and what is the mechanism behind it?