Organometallic Chemistry Flashcards
Explain Valence bond theory
To overlap half-filled valence orbitals to form covalent bonds in which the two electrons are shared between the bonding partners. This is not consistent with the dative bonding in coordination compounds where it is assumed that one partner donates an electron pair and the other partner accepts it.
What is Crystal field theory
The Crystal Field Theory (CFT) is a model for the bonding interaction between transition metals and ligands. It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand. The electrons in the d orbitals of the central metal ion and those in the ligand repel each other due to repulsion between like charges. Therefore, the d electrons closer to the ligands will have a higher energy than those further away, which results in the d orbitals splitting in energy.
What is the splitting affected by ?
- the nature of the metal ion
- the metal’s oxidation state (a higher oxidation state leads to a larger splitting)
- the arrangement of the ligands around the metal ion
- the nature of the ligands surrounding the metal ion
What is Crystal field stabilisation energy
The crystal field stabilization energy (CFSE) is the stability that results from placing a transition metal ion in the crystal field generated by a set of ligands. It arises due to the fact that when the d orbitals are split in a ligand field, some of them become lower in energy than before. For example, in the case of an octahedron, the t2g set becomes lower in energy. As a result, if there are any electrons occupying these orbitals, the metal ion is more stable in the ligand field by the amount known as the CFSE. Conversely, the eg orbitals are higher in energy. So, putting electrons in them reduces the amount of CFSE.
What does ligand field theory do ?
Ligand Field Theory looks at the effect of donor atoms on the energy of d orbitals in the metal complex.
What is Organometallic Chemistry?
Definition- A compound possessing a direct polar bond (Md+—Cd-) between the metal and one or more carbon atoms of an organic fragment (i.e., it must have an attached hydrogen atom, so not CO). It is at the interface of inorganic and organic chemistry
The Role of the Metal in Organometallic Chemistry
- Coordination of an organic fragment (e.g. an alkene) to a metal can modify its
reactivity, in a controlled manner (i.e. change reactivity patterns) - The metal can activate bonds in an organic substrate and act as a template to
facilitate bond formation in a manner not possible using conventional organic
methodology - The metal can stabilise highly reactive organic molecules such as carbenes
The Role of the Metal in Organometallic Chemistry
An organometallic compound is typically composed of…
An organometallic compound is typically composed of an organic fragment,
spectator ligands Ln (L = a neutral 2e ligand; n = integer) and an actor ligand X
The spectator ligand (Ln) is important as it modifies properties by..
Actor ligand?
Organic fragment?
(I) stabilising the metal complex
(ii) controlling solubility (and stability)
(iii) modifying the reactivity of the organic entity (electronic!) and controlling the
selectivity and efficiency (often steric) of the transformation
The actor ligand (X) is reactive and can be substituted by an incoming ‘reagent’ or
participate in a subsequent reaction
The organic fragment is bonded to the metal (sor s/p) and reactive/activated
Applications of Organometallic Chemistry
- Pharmaceuticals (the generation of single enantiomer compounds)
- Polymer industry- large scale synthesis
- Hydroformylation- A process that adds CO and H2 to an alkene
Ligand are often referred to as L or X type depending on the number of electrons they
donate to a metal (common for organometallic chemistry), explain each?
L type ligands are derived from charge neural precursors e.g. PR3, NR3. CO, alkene,
X type ligands are 1e radical donors when considered to be neutral e.g. Cl, H, R, OR
i.e. in the neutral method of electron counting or 2e donors when considered as derived from anion precursors e.g. Cl-, OR-, NR2-, H-,
R- i.e. in the oxidation state method of electron counting
In organometallic chemistry, the coordinated organic fragment is defined
in terms of its..
In organometallic chemistry, the coordinated organic fragment is defined
in terms of its hapticity (hn), where n = the number of carbon atoms
bonded directly to the metal centre as a single group.
Two distinct classes: n = even and n = odd
When n is even the hydrocarbon is considered to be a..
neutral donor, This is because the organic fragment is an electron pair donor from a filled p-orbital.
Hydrocarbons that bond through an even number of carbon atoms (n) always
donate n electrons (regardless of the electron counting model) as they have
closed shell electron configurations.
When n is odd the hydrocarbon must be considered to be an..
anion so that the fragment donates electrons in pairs.
Hydrocarbons that bond through an odd number of carbon atoms (n) are
considered as donating n+1 electrons in the ionic model as they must be anions
in order to donate electrons in pairs. In the neutral electron counting method
these organic fragments donate n electrons
Metal-carbonyls
Metal-hydrides
Metal Sigma Complexes
While metal carbonyl complexes are often thermodynamically
stable, CO is a also reactive fragment.
The metal-hydride (formally considered as H-) is a highly reactive
fragment and an intermediate in many industrial processes
these complexes involve an interaction between the metal
and the electrons in a H-X s-bond (where X-H = C-H, H-H, B-H, Si-H, P-H)
Metal Sigma Complexes
These complexes involve an interaction between the metal
and the electrons in a H-X s-bond (where X-H = C-H, H-H, B-H, Si-H, P-H). The bonding involves donation from the filled X-H s-bonding orbital and back-donation
into the empty H-X s* orbital and is very weak. This is because the s-bonding orbital is low in energy (and less basic) and the s* high
in energy and therefore less p-acidic
Dihydrogen as a ligand
Metal-dihydrogen complexes are intermediates in the cleavage of
H2 and have been well-studied in order to understand what
factors control the efficiency of H-H activation. The M-H2 bond is
very weak (ca. 9-10 kcal mol-1). Complexes of dihydrogen often
contain other p-acceptor ligands as well.
What are the two components to M-alkene bonding in CO
s-donation from the alkene and p-back-bonding from a filled d(pie) orbital
The stability of metal alkene complexes is due mainly to p-back-
bonding from a filled metal d-orbital to the empty p* of the alkene. In
the absence of this interaction the bonding is very weak
Metal-alkene orbital interactions
s-donation weakness (lengthens) the C-C bond as donation from the p-bonding orbital depletes bonding electron density in the C-C bond pie-back donation from the filled M-dp into the p* of the alkene also weakens the C-C bond (as it is C-C antibonding) but strengthens the M-C bonds (as this is a bonding interaction)
The strength of the metal-alkene interaction depends on the following (4)
(i) The nature of the metal-spectator ligand combination (basicity)
(ii) The alkene substituents
(iii) Steric interactions
(iv) Strain within the alkene
The nature of the metal-spectator ligand combination (2 limiting possibilities)
Electron-rich (basic) metal centres donate electron density into the C-C p* efficiently
which stabilises the metal-alkene bond (but weakens the C-C bond).
Oxidation state and ligand basicity are major factors.
Electron poor metals- alkenes are predominately s-donors to electron poor metal
centres (Lewis acidic). The M-alkene bonding in such complexes is weak as back-
donation is poor. This is most common for complexes that have high oxidation states or an overall positive charge
Alkenes bearing electron withdrawing groups bind more strongly due to better p-
back donation
Electronegative substituents (such as F,
CN and CO2R) lower the energy of p*
which results in more efficient back
bonding
Strained alkenes bind strongly
This is because coordination results in a change of hybridisation from sp2 towards
sp3 which relieves strain
Electron defficient metals have.. back donation
Electron rich basic metals have.. back donation
Poor back donation
Good back donation
Homolytic
Increasing electron density at the metal (basicity) results in increasing
back-donation from dpincreasing the H—-H distance from 0.85 Å to 1.65 Å and
ultimately H-H cleavage (i.e. homolytic cleavage)
Heterolytic
If M can stabilise the hydride, then M-H2 will be highly acidic and a base
could deprotonate the coordinated H2 to afford M-H and H+ (i.e. heterolytic cleavage)
Addition of M-H across an olefin (hydrometallation)
involves cleavage of a M-H bond and
formation of M-C and C-H s-bonds in a syn manner to give a metal-alkyl
Regioselectivity
For unsymmetrical olefins we must consider regiochemistry,
which way the M and H add to an unsymmetrical multiple bond
Addition of an organic electrophile and leaving group to an electron-rich metal
This is closely related to the SN2 nucleophilic displacement by the metal (type 2) but in this case the leaving group also adds to the metal (a formal insertion into R-Br).
Common for electron rich 16-electron complexes (as the electron count increases by 2). Involves formation of 2 new bonds to the M (i.e. M-C and M-Br in the above example) This transformation may be accompanied by ligand dissociation to maintain a ‘sensible’ electron count
Removal of carbon monoxide can be accomplished by either:
(a) Thermal reaction
(b) Chemical removal by reaction with an amine oxide (useful for strong M-CO bonds).
(c) Photochemical (promotion of a dp electron into an M-L s* orbital (antibonding)
which weakens the bond and facilitates substitution.
Removal/substitution of an anionic ligand (e.g. halide).
Abstraction of halide is generally assisted by a silver salt such as AgBF4 or AgSbF6 (the choice of anion is important as it will influence solubility and crystallinity)
Removal of Br- reduces the electron count by 2 units and
leaves a vacant site (orbital) for binding of the alkene or other 2e donor fragment
This reaction also leaves a positive charge on the complex; this is potentially useful
What is transmetalation?
introduces Anionic organic fragments on to a metal
The introduction of an anionic organic fragment such as R-, Ar- etc onto a transition
metal centre is a key process for their use in ‘organic synthesis’ This involves substitution of an anionic leaving group (X-) by a nucleophilic organic
group (Ar, R-) that is part of a main group metal complex MG-R/MG-Ar
Neutral organic fragments are typically introduced onto a metal by substitution, This may be an associative or dissociative process.
Explain both
Dissociative- occurs for 18e complexes, affected by strength of bonds to leaving ligand (trans effect important) and favoured by steric congestion at M.
Associative- common for 13-16e complexes, favoured for more basic incoming
ligands and more electrophilic metal centres, also favoured for sterically accessible
metal centres.
What is Oxidative addition and how does it affect bonding?
an overall process that cleaves a bond in the substrate
(A-B) and forms 2 new s-bonds to M which are then available for further transformation
a) Involves insertion of LnM into a s-bond of substrate A-B
b) M-A and M-B s-bonds are formed AND the A-B s-bond is cleaved
c) Formally raises the oxidation state of the metal by 2 units
d) Increases the electron count of the metal by 2 units
e) Is an equilibrium, the reverse reaction corresponds to reductive elimination
Oxidative addition of H2
- The electrons in the s-bond of H2 donate electron density to the empty metal s-
orbital (this weakens the H-H bond as the bonding electron density is depleted by
donation) - The electron rich metal donates dpelectrons into the s* orbital of H2 (this also
weakens the H-H bond as it involves donation into an anti-bonding orbital) - Good p-back donation from M into H-H s* is essential for cleavage of the bond
- Inefficient back donation can result in coordination of H-H to the M without cleavage
Oxidative addition to the 14e intermediate is much
faster than to 16e species
A Brief Recap: Oxidative Addition
Favoured for electron rich metals
Substrates: Non-polar (e.g. C-H, H-H, B-H, Si-H etc) and polar (C-Br, C-I, O-H)
Non-polar: oxidative addition involves s-donation and p-back bonding (see below)
Polar: SN2 type mechanism
Nucleophilic oxidative addition
s-bond metathesis (i.e. exchange of sigma bonds)
When oxidative addition and reductive elimination cannot occur or is unfavorable [e.g d0 Zr(IV) and Sc(III)] an alternative pathway can operate which involves addition of the non-polar C-H or H-H bond across an M-R bond to generate a product containing a new M-C bond
What is Oxidative Coupling
Oxidative coupling is a type of oxidative addition that couples alkynes, alkenes and
other unsaturated hydrocarbons to give metalacycles.
Most common for highly basic electron rich metals such as Zr(II), Ta(III), Ni(0), Pd(0),
Ir(I) and Rh(I).
What is Migratory Insertion?
a key bond forming reaction involving migration of a s-bonded
fragment to a coordinated unsaturated ligand (a formal insertion)
Migratory insertions involving ligands bonded through a single atom
e.g. carbon monoxide (CO)
Factors that affect the rate of insertion (4)
What is hydrometalation?
Insertion of an olefin into a M-H bond
What is carbometalation.
Insertion of an unsaturated substrate into a M-C bond
The rate of insertion into a M-C bond is slower than the corresponding insertion into a M-H bond.
However, insertion of an olefin into a M-C bond has a slightly larger thermodynamic
driving force than the corresponding insertion into M-H.
