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Year 2 - Chem > Organometallics > Flashcards

Organometallics Flashcards

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1
Q

Define an organometallic compound

A

Compounds containing at least one direct metal-carbon bond

Field also contains metal hydrides, amides, and other compounds

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2
Q

What is the bonding type in organometallics?

A

Mostly covalent
Best described using MO theory

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3
Q

What is hapiticity?

A

How many C atoms of a ligand are coordinated to a metal
η^x where x is number of C coordinated

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4
Q

How can the hapicity of a allyl ligand change?

A
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5
Q

How can the hapicity change for an indenyl ligand?

A
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6
Q

How are bridging ligands defined?

A

μx where x is the number of metal centres bridged

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7
Q

What is the structure of Co2(CO)8?

A
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8
Q

What is the 18 electron rule?

A

Kinetically stable organometallic complexes require:
Sum of neutral metal valence e- + sum of e- donated by ligands = 18

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9
Q

What are the exceptions to the 18 electron rule?

A

d8 square planar complexes
They use the 16 e- rule

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10
Q

Which complexes obey 18 e- rule best?

A

dn > 2
Strong π-acceptor ligands

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11
Q

What is the equation for formal oxidation state?

A

Formal oxn state = Charge on complex - sum of charges formally assigned to ligands (on removal from metal)

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12
Q

What is the equation for dn electron count?

A

dn electron count = metal group number - formal oxn state

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13
Q

What is the e- count and formal ligand charge of a ligand?

A

e- count = number of e- donated to ligand in M-L bond
formal ligand charge = change to charge on metal if ligand removed

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14
Q

What is a Lewis base?

A

Both e- in M-L σ bond derived from ligand
Each is a 2e- donor

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15
Q

What occurs when lewis bases are removed from metals?

A

Pair of e- remain with ligand
As donor atoms more electronegative than a metal
Therefore are neutral ligands

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16
Q

How do 1e- donor ligands change formal charge?

A

M-L bond requires one e- from M
So M “loses” an e- to ligand-based M-L bonding MO
Causes oxn state to increase by 1

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17
Q

How do amide and alkoxide ligands bind to a metal?

A

sp2 or sp hybridised so lone pair in low energy 2p orbital
π-donor ligands
Usually 3 or 5 e- donors but 1- ligands

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18
Q

What occurs to planar cyclic hydrocarbon ligands?

A

Donate/receive e- to become aromatic
This effects oxn state of M

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19
Q

What is a fischer carbene?

A

2e- donor CR2
Neutal ligand due to substituents and metal

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20
Q

What is a Schrock alkylidene?

A

2e- donor CH2, 2- ligand
Both donate 2e-, back donation is the difference to Fischer carbene

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21
Q

How can a NO ligand bonded to a metal?

A
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22
Q

How to calculate VE count of a metal complex?

A

Group number + e- donated by ligands in neutral forms +/- e- for anionic/cation charges
Then consider M-M bonds if sufficient d e-, by 18e- rule

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23
Q

How many MOs do TMs have and in a σ-only complex?

A

TMs:
9 valence MOs
5 x d, 1 x s, 3 x p

σ-only complex:
where x is number of L
x bonding, and x antibonding
9-x nonbonding with d character

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24
Q

What are class 1 complexes with reference to 18VE rule?

A

12 (d0) - 22 (d10) VE, due to weak field ligands and/or 3d metals.
t2g oritals effectively non-bonding and eg weakly antibonding

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25
Q

What are class 2 complexes wrt 18VE rule?

A

Up to and including 18VE (d0 to d6)
4d and 5d metals and/or strong σ-donor ligands
eg strongly antibonding, t2g non-bonding and can be used for π backbonding

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26
Q

What are class 3 complexes wrt 18VE rule?

A

Obey 18e- rule as long as no steric and/or electrostatic reasons
t2g are occupied π bonding and eg strongly antibonding σ*

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27
Q

What is the 16e- rule?

A

e- rich d8 complexes
Need strong σ donor ligands and/or 4/5d metals

4pz AO is non-bonding by symmetry, so only 8 bonding AOs

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28
Q

Do early transition metals follow 18VE rule?

A

Low # of metal VE so difficult to get to 18 without steric conflicts

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29
Q

What can sterically demanding ligands stabilise?

A

Otherwise highly reactive low e- complexes or coordination number

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30
Q

What is oxidative addition?

A

[M] + A-B <-> A-[M]-B

VE increases by 2
Oxn state increases by 2
d e- count decreases by 2

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31
Q

What is reductive elimination?

A

A-[M]-B <-> [M] + A-B

VE decreases by 2
Oxn state decreases by 2
d e- count increases by 2

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32
Q

What is migratory insertion?

A

[M]-A + B <-> [M]-B-A
VE, oxn state, and d-e- count constant

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33
Q

What are the types of migratory insertion?

A

1,1-insertion: η1 ligands such as CO
1,2-insertion: alkenes

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34
Q

What is a carbonyl insertion?

A

1,1-migratory insertion

35
Q

What is the alkene insertion?

A

1,2-migratory insertion of C2H4 with a metal hydride

36
Q

What is the stability of TM alkyls?

A

Thermodynamically stable
BUT need to be stabilised kinetically

Low activation energies of decomp reactions

37
Q

What is proof for the thermo stability of TM alkyls?

A

BDE is largely similar for all TMs in the same group

38
Q

How does the M-C bond change down the M group?

A

Increases down triad for TM
Decreases down group in main group

Due to more participation of d orbitals from 3d -> 4d -> 5d

39
Q

How does the M-C bond change across a series?

A

Lowers across the series

As d orbital contracts, so less ionic contributions

40
Q

How does M-C bond strength change with hydridisation of R?

A

increasing s character gives stronger bond

41
Q

How does M-C bond strength change with branching of alkane?

A

Primary > secondary > tertiary

Due to sterics

42
Q

When are briding alkyl/aryls present?

A

Electron-deficient compounds of main group metals

e.g. Li4Me4 or Al3Me6

43
Q

What type of bonding is present in bridging alkyl/aryl ligands?

A

3-centre-2-e- interaction

44
Q

What is agostic bonding?

A

e- deficient metals stabilised by intramolecular chelation

45
Q

When are α and β agostic interactions observed?

A

Intermediates or TS for α and β C-H elimination
Includes migratory insertion

46
Q

When are α and β agostics more common?

A

More common up the group as M-L bond strength is lower

47
Q

Why is TiEt4 unstable compared to PbEt4?

A

Ti has low energy vacant 3d AOs
Makes concerted bond-breaking/making possible by β-elim

48
Q

How can TM alkyls be kinetic stabilised by blocking vacant sites?

A

For 4-coordinate metal, bidentate donor ligand used
Doesn’t need to make Ti up to 18 VE, just block sites

49
Q

How can TM alkyls be kinetically stabilised by bulky alkyl ligands without β H?

A

Bulky groups increase stability
Also benzyl group stabilises via ipso interactions

50
Q

How can you synthesise metal alkyl compounds?

A

Transmetallation
Oxidative addition
From carbonylate anion & alkyl halide
Insertion into M-H bond

51
Q

What is a M-C ipso interaction?

A
52
Q

What is transmetallation?

A

Alkylation of main group halides

Uses:
Alkyl lithiums / grignards followed by alkyl aluminium and zinc

Alkylating power:
RLi > RMgX > AlR3 > ZnR2
Above trend due to covalency of M-C increasing, Zn only used when very oxidising M halides

53
Q

What is the mechanism for transmetallation?

A

Substitution
Eliminates MgCl or LiCl

54
Q

What is needed for transmetallation of WCl6?

A

WCl6 is too oxidising to use MeLi/grignards so use AlMe3 is used

Product is trigonal prismatic due to JT distortion

55
Q

What is oxidative addition?

A

Alkyl metal synthesis when more e- rich later metals with normally less than 18VE

Uses Me-I to attack via SN2 addition and a cationic intermediate (I or Cl as counterion)

Alternatively C-H bond activation called ortho-metallation

56
Q

What is ortho-metallation?

A

C-H bond activation reaction
Is oxidative addition

57
Q

How can metal alkyls be synthesised using a carbonylate and alkyl halide?

A

Electrophilic attack at e- rich M centre, which are stabilised by π-acceptor CO ligand

First reduction with Na, then MeI

58
Q

How can metal alkyls be made by insertion into metal-hydride bond?

A

Hydrides of group 6-10 stable, react with alkene and then 1,2-insertion reaction

59
Q

What reactions do metal alkyls react by?

A

α elimination
β-H elimination
Reductive elimination
Migratory insertion
σ bond metathesis reactions

60
Q

When does α-elim occur with metal alkyls?

A

Loses ligand, then α-elim occurs following by Nuc attack

Occurs when β-elim cannot

Forms Fischer carbene or Schrock alkylidene

61
Q

What is the mechanism of α-elimination of a metal alkyl?

A
62
Q

What requirements for metal alkyl reductive elim?

A

Groups elimited must be cis to each other

63
Q

What is the mechanism for metal alkyl reductive elim?

A
64
Q

What are σ-bond metathesis reactions with metal alkyls?

A

d0 TM compounds or lanthanides where OA cannot take place

Requires R-H and via 4-centre TS

65
Q

What is the mechanism for σ bond metathesis reactions with metal alkyls?

A

If R’ is H then stable, if an alkyl then could be in eqm

66
Q

What are some general features of CO complexes?

A

CO binds to TM in low oxn states
CO complexes almost always obey 18 VE rule
Used at starting materials

67
Q

What are the coordination modes of CO complexes?

A

Binds as terminal C-coordinated 2e- donor

68
Q

Why is IR used for CO complexes?

A

v(CO) intrinsically intense due to dipole moment changes and in free region

v(CO) sensitive to coordination (terminal or bridging) and electron-richness of M

69
Q

What is the structure of carbonyl complexes?

A

Obey 18VE rule

Exceptions such as V(CO)6, which are 17VE

If monomeric compounds not achievable then dimers/trimers/clusters with M-M bonds formed

M-M stronger down group so less likely to form CO-bridged compound

70
Q

What increases the chance of M-M bonds?

A

Stronger down the group as higher d-d overlap

71
Q

How are TM CO compounds made?

A

Direct with CO

If 0-valent bulk metals too unreactive, then reduce using Al/CO/H2

72
Q

What is the σ bonding of CO in TM complexes?

A
73
Q

What is the π-bonding of CO in TM complexes?

A
74
Q

What do CO ligands prefer to bond to and why?

A

Prefers to bond to medium to high d e-

π-bonding dominance causes it as e- populate the dπ-CO bonding MOs

75
Q

Why do CO complexes of main group metals not occur?

A

Only np AOs can be π-like in bonding but their interaction with the CO 2π (LUMO) is v poor

76
Q

How does IR of M-CO change across the period?

A

Increases wavenumber

As Zeff increases wavenumber increases
d-orbital increases in E so better backbonding & weakens CO bond

77
Q

What is nitrosyl compounds?

A

NO compounds

78
Q

What is the MO diagram of nitrosyl compounds?

A

Free NO has (π*)1 antibonding leading to formal 1e- transfer

79
Q

How are bridging ligands disruptive of M-M?

A

Orbitals for M-M used instead for metal-μ-bonding

80
Q

What is a case when bridging groups disrupt direct element-element bonding?

A
81
Q

What are the orbitals on N2?

A

MOs equally distributed on each N
Gives poorer M-N overlap for both σ-bonding and π-backbonding

82
Q

How does N2 vs CO2 bond to a metal?

A
83
Q

How does N2 bonding change down the group?

A

Better overlap down the group of metals
More bonding and backbonding, so nitrogen-nitrogen triple bond weakened

Year 2 - Chem (23 decks)

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