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CHEM2007 - General Inorganic Chemistry > Organometallic Chemistry > Flashcards

Organometallic Chemistry Flashcards

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1
Q

What is an organometallic complex?

A

A complex that contains at least one metal-carbon bond between one or more carbon atoms of an organic group or molecule and a main group, transition, lanthanide or actinide metal atom.

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2
Q

What effect do π acceptor (acid) ligands have on ΔO?

A

ΔO is increased as the electron density is transferred to ligand π* orbital so the t2g set is lowered in energy.

These are strong field ligands such as CO, PR3 and CN-.

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3
Q

What effect do π donor (basic) ligands have on ΔO?

A

ΔO is decreased as the electron density is transferred from ligand π orbital or lone pairs so the t2g set is raised in energy.

These are weak field ligands such as Cl, O and F.

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4
Q

What does ‘a’ mean?

A

Singly degenerate, symmetric with respect to rotation around the principal axis.

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5
Q

What does ‘b’ mean?

A

Singly degenerate, antisymmetric with respect to rotation around the principal axis.

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6
Q

What does ‘e’ mean?

A

Doubly degenerate.

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7
Q

What does ‘t’ mean?

A

Triply degenerate.

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8
Q

What does ‘g’ mean?

A

Centre of symmetry, even with respect to inversion.

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9
Q

What does ‘u’ mean?

A

No centre of symmetry, odd with respect to inversion.

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10
Q

What is included in the early metals?

A

Groups 1, 2 and 3 including the Lanthanides and Actinides.

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11
Q

What is included in the mid-metals?

A

Metals in the middle of the d-block (roughly group 4-7).

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12
Q

What is included in the late metals?

A

Metals after group 7.

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13
Q

What is a σ bond?

A

A bond that has zero nodal planes including the bond axis.

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14
Q

What is a π bond?

A

A bond that has one nodal plane including the bond axis.

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15
Q

What is a δ bond?

A

A bond that has two nodal planes including the bond axis. This is caused by two d-orbitals.

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16
Q

What does ηn mean?

A

The number of carbon atoms that are bound to a single metal atom.

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17
Q

What does μn mean?

A

The number of metal atoms that are connected to a single bridging carbon atom.

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18
Q

What are the four main types of organometallic complexes?

A

Ionic (charge separated)

Electron deficient

σ-bonding only

π-bonding

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19
Q

What are ionic (charge separated) organometallic complexes?

A

These form with only the most electropositive elements and are highly reactive and unstable. They are more stable if the charge can be stabilised by delocalisation.

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20
Q

What are electron deficient organometallic complexes?

A

Occurs when there are insufficient electrons to fill valence orbitals and form 2-centre-2-electron bonds between all atoms. This results in multi-centre bonding between R and two or more metal centres.

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21
Q

What are σ-bonding only organometallic complexes?

A

Occurs with closed-shell transition metal and main group centres, often leading to volatile compounds.

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22
Q

What are π-bonding organometallic complexes?

A

The interaction of π and π* orbitals of organic ligands with metal based orbitals.

This is especially prevalent with transition metal and zero valent lanthanides.

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23
Q

What is coordinative unsaturation?

A

Compounds which have less than an 18-electron count are coordinatively unsaturated and there may be a tendency to add further ligands.

This is because there must be one or more vacant orbital available.

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24
Q

What is coordinative saturation?

A

Complexes that have an 18-electron count are coordinatively saturated. For any reactivity to occur, they will most likely lose a ligand first.

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25
Q

What is the Pauling electroneutrality principle?

A

Neutral molecules, or those with a ±1 or ±2 charge, are more likely to form than highly charged species.

This is because the greater the charge, the greater the tendency to react.

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26
Q

What is the isolobal analogy?

A

Two fragments, often an organometallic fragment to an organic fragment, are related. They are deemed isolobal if the:
number,
symmetry properties,
approximate energy and,
shape
of their frontier orbitals as well as the number of electrons occupying them are similar.

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27
Q

What is an important consequence of the isolobal analogy?

A

Molecules constructed from isolobal fragments are themselves isolobal.

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28
Q

What is isolobal to a CH3 group?

A

A d7ML5 group.

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29
Q

What is isolobal to a CH2 group?

A

A d8ML4 group.

30
Q

What is isolobal to a CH group?

A

A d9ML3 group.

31
Q

How does a M-C bond compare with M-N, M-O or M-Halide bonds?

A

M-C bonds are weaker.

32
Q

What are the general trends in M-C bond strength?

A

The strength decreases with increasing atomic number of the carbon-like atom. For example, M-Si and M-Ge bonds are weaker than M-C. This is due to larger, more diffuse orbitals decreasing overlap.

The strength increases down the group for the transition metals. For example, Zr-C is stronger than Ti-C.

33
Q

Why are transition metal alkyls less stable than lead alkyls?

A

The difference in stability is due to kinetic reasons rather than thermodynamic.

34
Q

How does tetraethyl lead (PbEt4) decompose?

A

M-C bond homolysis (a high energy process) as Pb has a full octet, its 5d shell is filled, and the 6d and the σ* levels are energetically inaccessible.

This forms two radical species.

35
Q

How do transition metal organometallics decompose?

A

They decompose by concerted which is lower in energy, β-or reductive elimination.

36
Q

How can decomposition be prevented in transition metal complexes?

A

Use ligands stable to β-hydride elimination.

Bridgehead alkyls prevent decomposition as the resulting bridgehead alkene is unfavourable (Bredt’s rule).

Formation of arynes from arenes can be prevented by using ortho-substituents which are ≠H.

Use bulky ligands.

Saturate the coordination sphere.

Obey the Pauling Electroneutrality Principle.

Make 18-electron compounds.

37
Q

What ligands are stable to β-hydride elimination?

A

CH3, CH2Ph, CH2CMe3 and a range of methyl silanes.

38
Q

How can the coordination sphere be saturated?

A

By either:

Using extra ligands;
Using ligands which occupy more than one coordination site;
Use ligands with pendant coordinating groups;
Use sterically demanding ligands.

39
Q

How can the Pauling electroneutrality principle be obeyed?

A

The less charged a species a species is, the less reactive it will be.

40
Q

Why does the 18 electron rule (or 16 for square planar species) result in stability?

A

All of the s, p and d orbitals have been filled.

41
Q

How are the electrons arranged in an MO diagram for an 18-electron species?

A

The bonding and non-bonding orbitals are full but the anti-bonding orbitals are empty.

42
Q

What are the two methods for electron counting?

A

Covalent model.

Ionic charge model.

43
Q

What are exceptions to the 18-electron rule?

A

Small Δ ligands (anti-bonding orbitals lower in energy).

Low dn count, i.e. high oxidation state (hard to get enough ligands around the metal).

Electron rich d8 metal complexes (avoid populating the dx2-y2 orbital.

44
Q

Why do some ligands, such as alkoxide, have more than charge?

A

Depending on the electron-deficiency of the metal, two electrons from a lone pair (p-orbital) can be donated.

45
Q

What are the nine principal reaction types?

A

Salt elimination (salt metathesis).

Protonolysis.

Oxidative addition.

Reductive elimination.

Oxidative coupling.

Reductive cleavage.

Migratory insertions.

Elimination reactions.

σ-bond metathesis.

46
Q

What is the opposite of oxidative addition?

A

Reductive elimination.

47
Q

What is the opposite of oxidative coupling?

A

Reductive cleavage.

48
Q

What is the opposite of migratory insertions?

A

Elimination reactions.

49
Q

What is a salt elimination?

A

When a metal halide reacts with a group one alkyl to form a metal alkyl group and a salt.

For example:

M-Cl + Li-R → M-R + LiCl

This reaction is driven by the lattice enthalpy of the salt formation.

50
Q

What is protonolysis?

A

When a metal alkoxide, amide or alkyl reacts with anything with an acidic hydrogen to form a M-N bond.

For example:

M-R + HNR2 → M-NR2 + RH

This reaction is driven by the formation of the stronger M-N bond and the formation of a volatile RH.

51
Q

What is oxidative addition?

A

The simultaneous addition of two ligands into the coordination sphere of a metal. It is an overall process that has no mechanistic implications.

For example, the synthesis of Grignard reagents, RMgX from Mg and RX.

52
Q

What does oxidative addition require?

A

Non-bonding electron density at the metal.

A vacant coordination site.

A metal with accessible oxidation states separated by two units.

This makes it most common for d8 and d10 metals.

53
Q

What changes during an oxidative addition?

A

The oxidation state of the metal increases by 2.

The coordination number increases by 2.

The number of valence electrons increases by 2.

The dn decreases by 2.

54
Q

What favours oxidative addition?

A

Heavier elements of a group.

When the metal is electron rich (π-basic).

When hard or strong σ-donor ligands are present.

Small ligands are present.

The metal is large.

For strong M-A and M-B and weak A-B bonds.

55
Q

What are the four mechanism types for oxidative addition?

A

Concerted

SN2

Ionic

Radical

56
Q

What is reductive elimination?

A

The reverse process of oxidative addition.

Only works for cis-ligands.

57
Q

What is oxidative coupling?

A

It is a special case of oxidative addition generally involving an early transition metal with a d2 configuration. These complexes are easily oxidised because the occupied orbitals are high lying. The coupling of the two ligands creates a metallocycle.

58
Q

What is reductive cleavage?

A

The reverse of oxidative coupling.

Metal complexes involved in oxidative coupling and reductive cleavage are often in equilibrium.

59
Q

What is a migratory insertion reaction?

A

An R ligand on the metal moves to a different ligand and a new ligand attaches to the metal.

60
Q

What are four types of elimination reactions?

A

α-hydride elimination

α-hydride abstraction

β-hydride elimination

γ-, δ-, ε- eliminations.

61
Q

What is α-hydride elimination?

A

The transfer of a hydride from the α-position on a ligand to a metal centre.

Formally, this is a type of oxidative addition reaction and therefore α-elimination cannot occur in a d0 or d1 metal complex.

62
Q

What is α-hydride abstraction?

A

For d0 or d1 metal complexes, the α-hydrogen transfers to an adjacent ligand rather than the metal centre.

63
Q

What is β-hydride elimination?

A

The transfer of a hydride from the β position on a ligand to the metal centre.

The complex must be coordinatively unsaturated for β-hydride elimination to occur and the metal must usually have less than 18 electrons.

64
Q

What are γ-, δ-, ε- eliminations?

A

Eliminations occurs from the γ-, δ- or ε- position and transfer to the metal centre.

This is like an α-elimination.

65
Q

What is σ-bond metathesis?

A

Two sigma bonds break and form two new sigma bonds with opposing substituents.

For example:

M-R + A-B → M-A + R-B.

This reaction pathway usually occurs when oxidative addition and reductive elimination are not viable mechanistic reaction pathways.

66
Q

The Art of Organometallic Synthesis:

Is the metal early or late?

A

Early - salt elimination or protonolysis.

Late - salt elimination or consider redox route.

67
Q

The Art of Organometallic Synthesis:

Group 1 and a halide present?

A

If a salt can be formed, salt elimination is a good pathway.

68
Q

The Art of Organometallic Synthesis:

Can ligand scrambling occur?

A

Substitute bulky ligands on first.

Early - polar, ionic bonds, little orbital control, labile ligands.

Late - covalent bonds, large orbital control, may be regarded as static.

69
Q

The Art of Organometallic Synthesis:

Are chemical redox reactions required?

A

Oxidation - use Ag(I) reagent or Cp2Fe reagent.

Reduction - use alkali metal or other electropositive metals with low ionisation potentials or Cp2Co.

70
Q

The Art of Organometallic Synthesis:

Is the metal electron rich and can its oxidation state be changed by +2?

A

Consider oxidative addition of a polar bond such as C-X or Si-H.

71
Q

The Art of Organometallic Synthesis:

Can a neutral ligand displace another neutral ligand easily to make a precursor?

A

Hydrides - use α- or β- elimination reaction.

Acyls - use 1,1 migratory insertion.

Carbenes - α-abstraction (Schrock), RLi/R3OBF4 treatment of a carbonyl (Fischer).

72
Q
A

CHEM2007 - General Inorganic Chemistry (6 decks)

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