Mechanisms Flashcards
What is ligand substitution?
Where one Lewis base replaces another at a metal centre
e.g.
the reaction of hydrated Co^II with chloride, [Cl]–
Define “nucleophilicty”
“the rate of attack on a complex by a base compared with the rate of attack by
a standard base”
* the rates of substitution span a wide range
What are features of the eighteen electron rule?
- Many organometallic complexes, particularly those with p-acceptor ligands (e.g. CO, ethene) have an electron count of 18 (provides a very useful way of predicting the stoichiometry of organometallic complexes)
The logic of the 18-electron rule is easily demonstrated in the case of an octahedral complex like Cr(CO)6:
- 6 x metal orbitals which lie along the cartesian axes, dz2 dx2–y2 px py pz and s orbitals interact with the 6 x ligand s-orbitals to form 12 x molecular orbitals (MOs)
- 6 x MOs are low energy bonding orbitals and 6 x MOs high energy antibonding orbitals
- also three metal orbitals which point between the ligands: dxy dxz and dyz (these are non-bonding and of intermediate energy)
- take the 6 x bonding orbitals 3 x non-bonding orbitals = 9 x orbitals, which have the capacity to accommodate 18 electrons - There are plenty of exceptions to the 18-electron rule, but almost always involving fewer electrons rather than more
Important to our discussion of hydroformylation is the systematic preference of the heavier d8 metals Rh(I), Ir(I), Pd(II) and Pt(II) for 16-electrons
What are features of the associative mechanism?
- Incoming ligand (Y) attacks the original complex to form an intermediate of higher coordination number before the departing ligand (X) leaves
- Large energy gap between the non-bonding orbitals and the anti-bonding orbitals means it is difficult for organometallic complexes to exceed the 18-electron rule (would require placing electrons in a high energy orbital)
- Associative mechanism requires both the leaving ligand and the attacking ligand to be attached to the metal at the same time, thereby exceeding the 18 electrons
- Associative mechanisms are therefore not favoured by organometallic complexes
- 16-electron complexes usually react by an associative mechanism, which involves an 18-electron intermediate
(unless, for special reasons, a way is found to circumvent the 18-electron rule)
Show how the associate mechanism works
- Two-step mechanism
- An intermediate is formed on coordination of entering group to the original complex
The CN of the intermediate is higher than in the original complex, which must therefore be coordinatively unsaturated, or be capable of becoming so…
What are features of the dissociative mechanism?
- The departing ligand (X) leaves first giving an intermediate with a reduced coordination number, that is attacked by the is incoming ligand (Y)
- A dissociative mechanism involves the initial breaking of a bond, which can be a high-energy process
- Nevertheless 18-electron organometallic complexes usually undergo ligand exchange by a dissociative mechanism involving a 16-electron intermediate
Show how the dissociative mechanism works
- also a two-step mechanism
- An intermediate is formed on loss of the leaving group
How do rate and angles indicate D or A mechanism?
What are features of the concerted or interchange mechanism and show how it works?
- A one-step mechanism
- An activated complex (“transition-state” complex) is formed, but is not a true
intermediate (i.e. there is no possible way of isolating, trapping, visualizing etc.) - common for metals with CN 6
- Distinction from associative mechanism depends on whether the intermediate persists long enough to be detected
What rate constant properties indicate whether the process is A (Ia)?
If rate constants for the formation of the activated complex depend strongly on the properties (electronic and streric) of the entering group Y:
We infer that the activated complex has a strong bond to Y:
Therefore we have an associative (a) process
What rate constant properties indicate whether the process is D (Id)?
If the rate constants are little influenced by the properties of Y:
Then the rate of the overall reaction must be controlled by the rate at which the M–X (X = leaving group) can break:
Therefore we have dissociative (d) process
What do we view interchange of in terms of stoichiometric mechanisms?
Lying between the two extremes of A and D stoichiometric mechanisms.
What is a stoichiometric mechanism?
The sequence of elementary steps by which the reaction takes place
What is an intimate mechanism?
The details of the activation processes and energetics of formation of an activated complex in the rate determining step
Summary of D mechanism?
- the M–X bond is broken before the entering group Y attaches
- the coordination number of M is decreased by one in the transition state (this mechanism is similar to the SN1 used in organic chemistry)
What are features of migration reactions?
(also called Insertion or Migratory Insertion Reactions)
- important because it represents the step in which the metal promotes a transformation of the ligands which would not occur in the absence of the metal
- two examples in the hydroformylation reaction:
i) hydride migrates to coordinated alkene forming an alkyl ligand
ii) alkyl migrates to coordinated CO forming an acyl ligand - both reactions very common and important in other catalytic and stoichiometric reactions
More features of mirgation reactions:
- key requirement for these reactions to occur is the two ligands should be in cis-positions
- a consequence is that the new species formed by the migration is coordinatively
unsaturated (i.e. 16-electron) and is able to take up another ligand - in a good catalytic process it is very difficult to collect information on the reactions
because the intermediates have very short lifetimes - for this reason chemists often study model systems where the reaction rates are more
manageable - an interesting model for the alkyl migration reaction is provided by [MnMe(CO)5]:
NOTE:
- the alkyl migration is reversible
- a “vacant coordination site” is generated as the alkyl group migrates - this site is readily filled by a new ligand
What are the features of oxidative addition?
- the central atom is oxidized with a simultaneous increase in the coordination number
- the significance for organometallic catalysis is that it offers a mechanism for the activation of otherwise unreactive species
e.g. - the H–H single bond is very strong (bond dissociation energy is 435 kJ mol-1) - high temperatures are often required to initiate reaction - however many transition metal species will cleave dihydrogen in a concerted
reaction with low activation energy - once the H atoms are separated in the coordination sphere of the metal they
may migrate to other ligands (as seen in the hydroformylation process)
Describe mechanism of Iridium reacting with dihydrogen
Vaska’s complex
- Coordination number has changed from 4 to 6
- good evidence that the detailed mechanism of activation involves the interaction of the s-bond electrons of H2 with an empty metal orbital and simultaneous back-donation from filled metal dp orbitals into the s*orbital of H2
- filling the s* orbital weakens the H–H bond as the M–H bonds increase strength so that the hydrogens separate smoothly in a concerted manner to form two hydride ligands
- the side-on concerted mechanism is not the only possible mechanism by which oxidative addition reactions can proceed
- ionic and radical mechanisms are also known
- which mechanism the reaction follows is determined by the nature of the metal complex, the reactant molecule and the solvent
What are the features of the ionic mechanism?
- requires the metal centre to act as a nucleophile
- the reactant molecule is typically an alkyl or benzyl halide and the reaction may be
regarded as an SN2 displacement at the carbon centre with the metal complex acting
as the nucleophile - a two-step process: note that the addition is usually trans although cis addition is possible
if the intermediate undergoes rearrangement - this mechanism shows typical characteristics of an SN2 reaction
e.g. -itproceedsfasterinpolarsolvents - if the carbon centre is chiral, the reaction proceeds with inversion
Draw the catalytic cycle for synthesis of acetic acid
