Organometallic Chemistry Flashcards
(54 cards)
1
Q
Describe carbon ligands in terms of field strength, donor ability and acceptor ability. Give examples of specific ligands.
A
- Strong field ligands so produce large splittings
- Either good sigma donors e.g. carbanions like -CH3 (give big value of delta)
- OR good pi-acceptors e.g. CO (metal donates e- density into CO pi* bond, lowers energy of metal t(2g) set)
- High energy donors/low energy acceptors = good match with metal d-orbitals
2
Q
How are valence electrons counted? What is the “magic number” for organometallic complexes?
A
d-electrons in metal + (no.of ligands x LPs)
E.g. [Fe(H2O)6]3+
Fe3+ = 8 - 3 = d5
6 x H2O:–> = 12
Total: 17 valence electrons (VE)
Magic number = 18VE
3
Q
What is the 18VE rule? When can it be broken?
A
- Stable organometallic complexes are typically 18VE
- Can be broken if:
- Other geometries
- Weak-field ligands
- Early transition metals (not enough d-electrons to make it to 18VE)
4
Q
What is ligand hapticity and how is it written?
A
- Number of atoms a ligand binds to metal through
- Written as eta (η):
- eta 1: C=C
- eta 2: C=C-C
- eta 3: C=C-C^-
- eta 4: C=C-C=C
- eta 5: cyclopentadienyl “Cp-“
- If no eta notation, assume max. hapticity for cyclic ligands
- E.g. eta 1: only -ve charge contributes (2e-)
eta 3: -ve charge + double bond contribute (4e-) - eta 5: -ve charge + both double bonds contribute (6e-)
5
Q
What is the 16VE rule?
A
- Square planar complexes have:
- Fewer ligands than octahedral
- Different MO scheme (1 inaccessible orbital rather than 2)
- So they don’t obey 18VE rule
6
Q
What is the effect of high/low energy metals on the strength of C-O bonds?
A
Low energy metal will slightly strengthen bond
High energy metal will weaken bond
7
Q
Describe sigma donation and what the effect is on the strength of the C-O bond?
A
- CO ligand delocalises (stabilises) the CO antibonding electrons
- This strengthens the CO bond as metal can back donate electrons into pi* orbitals of CO (pi backbonding)
- Synergistic effect: sigma donation and pi backbonding work together to strengthen metal CO bond
8
Q
Describe pi accepting and the effect it has on the strength of the CO bond?
A
- Metal populates orbital with CO antibonding character
- Weakens CO bond as it shifts e- density away from C-O bond into pi* orbital (destabilises C-O bond)
9
Q
how does frequency relate to force constant?
A
v is directly proportional to sqrt(k/mu)
10
Q
What is the effect of bond strength on the force constant?
A
- Strong bond has large force constant
- In carbonyl complexes, C-O bond gets weaker down sequence (frequency decreases –> weakens CO bond)
11
Q
What are the general effects of electron rich/poor metals on CO frequency and bond strength?
A
- Electron rich metals donate a lot of e- density into CO, weakening bond and lowering frequency
- Good sigma donors make metal more electron rich
- Electron poor metals do this much less
- Good pi acceptors make metal more electron poor
12
Q
How do you make a metal-alkene complex?
A
Dative bond from double bond of ethene to the metal
13
Q
What is synergistic bonding?
A
sigma bonding to the metal
pi bonding from the metal
one mode reinforces the other
14
Q
What is the difference in bonding when sigma bonding dominates vs when pi bonding dominates?
A
Sigma:
C=C double bond donating to metal
Pi:
If back-bonding so strong that CC pi* orbital is fully populated, pi bond between C atoms obliterated
Resonance forms
15
Q
Give an example of strong backbonding in a complex
A
- [Ru(C2H4)(PMe3)4]
- PMe3 = good sigma donor ligands so Ru centre = very electron rich (so lots of backbonding
- e- rich metal centres donate e- density to carbonyl ligand
- IMPORTANT: free ethene C=C is 134 pm but in complex is 144 pm
- Due to metal populating CC pi*, weakening CC bond, lengthening CC bond
16
Q
Give an example of weak backbonding in a complex
A
- K[Pt(C2H4)Cl3]
- Poor backbonding because bulky groups (Cl, poorer sigma donor than phosphine) adopt orientations which prevent backbonding
- free ethene is 134 pm, in complex is 138 pm (very similar)
- sigma mode dominates, preserving C=C character
- bond angle is 120 (sp2-like so similar to ethene)
17
Q
What are the main reactions of metal-alkene complexes?
A
- Substitution at metal, R2PdCl(Me) + LL –> L2PdCl(Me)
- OR Nuc. attack at alkene:
- In metal ethene complexes, you can activate double bond (better electrophile than free ethene)
- Similar to Sn2 reaction (very loose analogy)
- OR migratory insertion (polymerisation):
- Complex + alkene –> metal-alkene complex then intramolecular reaction where nuc. C group attacks double bond
- End up with complex similar to starting material
18
Q
What is a cyclopentadienide complex, give a typical example.
A
- C5H5- (Cp), has 6e-
- Ferrocene (FeCp2)
19
Q
How does cyclopentadienide bond? (In terms of dz^2 orbital)
A
- dz^2 is essentially non-bonding
- dz^2 has a node where psi = 0 and directly hits C 2p orbitals
- Overlap for dz^2 orbital is very poor with C ligand because of particular orientation of d orbital within molecule
20
Q
How does cyclopentadienide bond in terms of e1g orbital?
A
- Metal dxz and dyz point into Cp pi orbitals
- Major contribution to bonding
- Metal e1g = antibonding
- Both a bonding combination, in phase overlap (carbon) and antibonding combination, out of phase overlap (d-orbital)
21
Q
Why can metallocenes violate the 18VE rule?
A
- All of its orbitals are similar energy, so adding another electron is okay
- Metal d orbitals are not so bonding they must be filled, nor so antibonding they must be left empty
- But big enough gap that 18VE is somewhat favoured
22
Q
Describe the Friedel Crafts reaction of ferrocene
A
ferrocene + acyl chloride CH3COCl (+ AlCl3) –> ferrocene-COCH3
double bond from C=O in acyl chloride cleaves down, kicking out Cl to attack Al in AlCl3
Product = H3C-C(triplebond)O+ (potent electrophile)
If 2 equivs. acyl chloride added, product is the better nucleophile
23
Q
Explain metal hydride chemical shifts
A
- electron density shields proton from external magnetic field
- Hydride ligands have substantial H- character so highly shielded
- Typically negative shifts ( ~ -10 ppm)
24
Q
Why can H- ligand act as H+/acid?
A
It stabilises conjugate base (low oxidation state) by using pi acceptor ligands
25
Explain aromatic chemical shifts
- Aromatic ring current (movement of delocalised pi electrons in ring when subjected to external magnetic field) deshields protons in benzene
- Highly deshielded relative to alkenes
- Metal-ligand bonding interrupts ring current (aromatic electrons bond to metal)
- Metal arene shift typically lower than free benzene (due to circulating charge/pi electrons in ring)
- B0 = ext. magnetic field from massive magnet (NMR machine)
- Blocal = local magnetic field within molecule, adds to ext. field
26
Rationalise to 1H NMR shifts for MCp2 where M = Cr (4.1), Mo (4.6), W (4.9)
- Energy of orbitals increases from 3d-->5d
- E(orb) is directly proportional to (Zeff/n)^2
- As you go to bigger shell, electrons further from nucleus, less tightly held
- In aromatic ring, 6e- more distant in energy to each other down series
- So, extent to which ring current is disrupted gets smaller down series
- So ring current is still there to a larger extent down series as it's not stolen by metal ligand bond as strongly when energy match is poorer
27
What is the Grignard reaction? Give example with Mg
- Mg atoms inserted into metal-halogen bond to form Grignard reagent
- E.g. R-Br (+ Mg) --> R-Mg-Br (source of R-)
- Mg is oxidised from ox. state 0 to +2
- Mg adds to its coordination number (0 to 2)
- So, oxidative addition of RBr to Mg
28
When can H2 act as an oxidising agent instead of a reducing agent?
When H2 is more electronegative than the atom it is trying to oxidise e.g. Ir
(Typically less electronegative than atoms such as C)
29
What are the general requirements of a reactant in oxidative addition (OA)? What are not requirements but normally happen?
1. Metal must be able to increase ox. no. by 2
2. Metal must be able to increase coordination no. by 2 (starting material must have enough space around it to accommodate 2 more ligands)
- Normally cis addition
- Normally single reaction step
30
What is an agostic interaction?
When C-H bond acts as ligand to metal
Sigma bonding and pi backbonding
31
What is orthometallation?
Ortho position H atom is metallated (forms bond to metal)
Intramolecular oxidative addition reaction
32
What is reductive elimination (RE)?
- Metal reduced by 2 (ox. no. increases by 2)
- Metal loses 2 from coordination no. (elimination)
- E.g. reductive elimination of aldehyde from Rh
33
What is the strict requirement for RE?
Cis elimination (cis groups can eliminate)
as trans groups cannot eliminate
34
What are the general requirements for a reactant in RE?
- Metal must be able to decrease ox. no. by 2
- Metal must be able to decrease coordination no. by 2 (normally/often octahedral --> square planar)
- Eliminated ligands MUST be cis to each other (very few exceptions)
35
Give an electronic analysis of OA and RE
- Focuses on oxidation/reduction parts of reactions
- Good sigma donors stabilise high ox. states (favours OA reactions, hinders RE reactions)
- E.g. PMe3 (good sigma donor) will stabilise the higher ox. state product from OA but hinders RE (need weaker sigma donor for RE)
- Good pi acceptors stabilise low ox. states (favours RE, hinders OA)
- E.g. CO (good pi acceptor) stabilises low ox. state product from RE
36
Give a steric analysis of OA and RE
- Focuses on addition/elimination parts of reactions
- Bulky ligands often resist increases in coordination no. by crowding metal centre (hinders OA, favours RE)
- E.g. PtBu3 (bulky ligand) prevents addition of ligands (no room for more ligands)
- Metal centre often quite crowded (addition of substrate to terminal positions often sterically favoured)
- E.g. Addition (OA) of terminal C-H bond in n-pentane
37
What is migratory insertion (MI)?
Alkyl ligand migrates from metal to carbonyl ligand (inserts itself into it)
Accommodating ligand usually loses some pi bonding (movement of ligand breaks pi bond)
38
What are the general requirements of reactant in MI?
- Migrating ligand must be close to ligand it will insert into (so usually cis)
- Ligand receiving migratory group must be able to accommodate it (normally by losing pi bond)
39
Explain why the rate of MI reaction changes with M in the order Cr>>Mo>W
- Rate of reaction depends on breaking of pi bond (higher in energy than sigma bond)
- W should be slowest rate as W has best energy match to CO pi* orbital so has the strongest metal carbonyl pi bond
- Data given shows pi bonding must be stronger than sigma bonding
40
Propose a mechanism for MI (see OM lecture 6)
- Starting material = 18VE (stable)
- Heat it up, MI of methyl group onto one of carbonyl groups
- Gives 16VE compound (not enough electrons)
- Coordinate it with alkene to give 18VE compound
- Another MI of a different ligand (carbonyl) onto alkene to give 16VE compound
- O atom from carbonyl binds to metal (bidentate ligand) to give 18VE compound
41
What is hydride elimination?
- Hydride eliminated from organic fragment (carbon, instead of metal)
- Develop strong pi backbonding to unsaturated organic fragment, often favourable
- Doesn't have to be beta (alpha, gamma, delta, epsilon)
42
How are agostic interaction and beta hydride elimination (BHE) similar?
- Agostic bonding and hydride elimination both give the metal electron density
- Beta hydride = geometrically accessible (curls around/faces towards metal unlike alpha hydride)
43
What are the general requirements of reactant in BHE?
- Hydride must approach metal (geometry typically favours beta over alpha/gamma)
- Metal must be able to accommodate increase in coordination number (typically 1-2 higher than starting material)
44
What is a free (gas phase) carbene (CR2)? Describe its electronic configuration types and how it is observed wrt reactivity and bonding.
- "Half an alkene"
- :CR2 OR +/-CR2 notation
- Reactivity/bonding with pairs of e- observed, not radical
- 4 possible electronic configurations:
1. empty p-orbital, LP in sp2 orbital (IMPORTANT, observed)
2. 1e- in each orbital, same spin (not observed)
3. 1e- in each orbital, different spins (not observed)
4. Both e- in p orbital, different spins (not observed)
45
What are the 2 families of carbene complex? Describe them.
M=C(X)R
Fischer:
- X = OR, NR2, etc (contain LP)
- Typically electrophilic at C
Schrock:
- X = H,R
- Typically nucleophilic at C
46
What are Schrock carbenes (alkylidenes)?
- Metal typically from left of d-block (high in energy as they have low Zeff)
- Antibond = close in energy to metal d orbital so sits mostly on metal
- Ligand = 2p (low in energy as low value of n)
- Pi bond close in energy to C energy it comes from so pi bond resembles C more than metal and LP sits mostly on C
- C = nucleophilic
- CH2 ligand has 2- charge
47
How are Schrock carbenes synthesised?
Deprotonation (pulling protons off other species)
48
How do you count electrons of a CH2 ligand in a Schrock carbene?
CH2 ligand = 4e- donor (2 from LP, 2 from -2 charge)
If CH2-R, only 2e- as LP donating to R (C = nucleophilic)
49
What are Fischer carbenes?
- Metal typically from right of d-block (low in energy as high Zeff value)
- C p-orbital better considered as pi* orbital (high in energy so closer in energy to each other)
- Substantial metal d-orbital character in the M=C pi-bond (close in energy)
- Ligand = neutral (2e- donor)
- C = electrophilic
50
How are Fischer carbenes synthesised?
+OR3 used as a source of R+
Electrophilic attack at CO, CS or CNR ligand:
Nucleophilic attack at electrophilic C then R+ attacks O-
51
How are CR2 ligands electron counted in Fischer carbenes?
2e- from LP (no charge)
52
Compare/contrast Schrock, Fischer and NHC (cyclic 5 membered ring) carbenes
Energy gap between p and sp2 orbital (smallest to biggest, least stable to most stable): Schrock, Fischer, NHC
sigma effect: sp2 orbital falls in energy as X groups added
pi effect: C p-orbital gets higher in energy as X groups added
Schrock = CR2
Fischer = CR(OR)
NHC ("super Fischer carbene")= CN2
All have pair of e- in sp2 orbital
53
What is olefin (alkene) metathesis?
C atoms 'swap partners'
E.g. CHR1=CHR2 + CHR3=CHR4 --> CHR1=CHR3 + CHR2=CHR4
54
What is the mechanism for alkene metathesis?
- Uses Grubbs' catalyst (allows for alkene metathesis reactions)
Ru centre
- 18VE <--> 16VE
- Then coordinate alkene
- Schrock carbene reacts with alkene to make 4 membered ring (square intermediate)
- Intermediate can collapse to go back to starting material or to give Ru centre a different carbene (and alkene ligand a different substituent)
- All steps are reversible
