Organometallics Flashcards
Define an organometallic compound
Compounds containing at least one direct metal-carbon bond
Field also contains metal hydrides, amides, and other compounds
What is the bonding type in organometallics?
Mostly covalent
Best described using MO theory
What is hapiticity?
How many C atoms of a ligand are coordinated to a metal
η^x where x is number of C coordinated
How can the hapicity of a allyl ligand change?
How can the hapicity change for an indenyl ligand?
How are bridging ligands defined?
μx where x is the number of metal centres bridged
What is the structure of Co2(CO)8?
What is the 18 electron rule?
Kinetically stable organometallic complexes require:
Sum of neutral metal valence e- + sum of e- donated by ligands = 18
What are the exceptions to the 18 electron rule?
d8 square planar complexes
They use the 16 e- rule
Which complexes obey 18 e- rule best?
dn > 2
Strong π-acceptor ligands
What is the equation for formal oxidation state?
Formal oxn state = Charge on complex - sum of charges formally assigned to ligands (on removal from metal)
What is the equation for dn electron count?
dn electron count = metal group number - formal oxn state
What is the e- count and formal ligand charge of a ligand?
e- count = number of e- donated to ligand in M-L bond
formal ligand charge = change to charge on metal if ligand removed
What is a Lewis base?
Both e- in M-L σ bond derived from ligand
Each is a 2e- donor
What occurs when lewis bases are removed from metals?
Pair of e- remain with ligand
As donor atoms more electronegative than a metal
Therefore are neutral ligands
How do 1e- donor ligands change formal charge?
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
How do amide and alkoxide ligands bind to a metal?
sp2 or sp hybridised so lone pair in low energy 2p orbital
π-donor ligands
Usually 3 or 5 e- donors but 1- ligands
What occurs to planar cyclic hydrocarbon ligands?
Donate/receive e- to become aromatic
This effects oxn state of M
What is a fischer carbene?
2e- donor CR2
Neutal ligand due to substituents and metal
What is a Schrock alkylidene?
2e- donor CH2, 2- ligand
Both donate 2e-, back donation is the difference to Fischer carbene
How can a NO ligand bonded to a metal?
How to calculate VE count of a metal complex?
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
How many MOs do TMs have and in a σ-only complex?
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
What are class 1 complexes with reference to 18VE rule?
12 (d0) - 22 (d10) VE, due to weak field ligands and/or 3d metals.
t2g oritals effectively non-bonding and eg weakly antibonding
What are class 2 complexes wrt 18VE rule?
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
What are class 3 complexes wrt 18VE rule?
Obey 18e- rule as long as no steric and/or electrostatic reasons
t2g are occupied π bonding and eg strongly antibonding σ*
What is the 16e- rule?
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
Do early transition metals follow 18VE rule?
Low # of metal VE so difficult to get to 18 without steric conflicts
What can sterically demanding ligands stabilise?
Otherwise highly reactive low e- complexes or coordination number
What is oxidative addition?
[M] + A-B <-> A-[M]-B
VE increases by 2
Oxn state increases by 2
d e- count decreases by 2
What is reductive elimination?
A-[M]-B <-> [M] + A-B
VE decreases by 2
Oxn state decreases by 2
d e- count increases by 2
What is migratory insertion?
[M]-A + B <-> [M]-B-A
VE, oxn state, and d-e- count constant
What are the types of migratory insertion?
1,1-insertion: η1 ligands such as CO
1,2-insertion: alkenes
What is a carbonyl insertion?
1,1-migratory insertion
What is the alkene insertion?
1,2-migratory insertion of C2H4 with a metal hydride
What is the stability of TM alkyls?
Thermodynamically stable
BUT need to be stabilised kinetically
Low activation energies of decomp reactions
What is proof for the thermo stability of TM alkyls?
BDE is largely similar for all TMs in the same group
How does the M-C bond change down the M group?
Increases down triad for TM
Decreases down group in main group
Due to more participation of d orbitals from 3d -> 4d -> 5d
How does the M-C bond change across a series?
Lowers across the series
As d orbital contracts, so less ionic contributions
How does M-C bond strength change with hydridisation of R?
increasing s character gives stronger bond
How does M-C bond strength change with branching of alkane?
Primary > secondary > tertiary
Due to sterics
When are briding alkyl/aryls present?
Electron-deficient compounds of main group metals
e.g. Li4Me4 or Al3Me6
What type of bonding is present in bridging alkyl/aryl ligands?
3-centre-2-e- interaction
What is agostic bonding?
e- deficient metals stabilised by intramolecular chelation
When are α and β agostic interactions observed?
Intermediates or TS for α and β C-H elimination
Includes migratory insertion
When are α and β agostics more common?
More common up the group as M-L bond strength is lower
Why is TiEt4 unstable compared to PbEt4?
Ti has low energy vacant 3d AOs
Makes concerted bond-breaking/making possible by β-elim
How can TM alkyls be kinetic stabilised by blocking vacant sites?
For 4-coordinate metal, bidentate donor ligand used
Doesn’t need to make Ti up to 18 VE, just block sites
How can TM alkyls be kinetically stabilised by bulky alkyl ligands without β H?
Bulky groups increase stability
Also benzyl group stabilises via ipso interactions
How can you synthesise metal alkyl compounds?
Transmetallation
Oxidative addition
From carbonylate anion & alkyl halide
Insertion into M-H bond
What is a M-C ipso interaction?
What is transmetallation?
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
What is the mechanism for transmetallation?
Substitution
Eliminates MgCl or LiCl
What is needed for transmetallation of WCl6?
WCl6 is too oxidising to use MeLi/grignards so use AlMe3 is used
Product is trigonal prismatic due to JT distortion
What is oxidative addition?
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
What is ortho-metallation?
C-H bond activation reaction
Is oxidative addition
How can metal alkyls be synthesised using a carbonylate and alkyl halide?
Electrophilic attack at e- rich M centre, which are stabilised by π-acceptor CO ligand
First reduction with Na, then MeI
How can metal alkyls be made by insertion into metal-hydride bond?
Hydrides of group 6-10 stable, react with alkene and then 1,2-insertion reaction
What reactions do metal alkyls react by?
α elimination
β-H elimination
Reductive elimination
Migratory insertion
σ bond metathesis reactions
When does α-elim occur with metal alkyls?
Loses ligand, then α-elim occurs following by Nuc attack
Occurs when β-elim cannot
Forms Fischer carbene or Schrock alkylidene
What is the mechanism of α-elimination of a metal alkyl?
What requirements for metal alkyl reductive elim?
Groups elimited must be cis to each other
What is the mechanism for metal alkyl reductive elim?
What are σ-bond metathesis reactions with metal alkyls?
d0 TM compounds or lanthanides where OA cannot take place
Requires R-H and via 4-centre TS
What is the mechanism for σ bond metathesis reactions with metal alkyls?
If R’ is H then stable, if an alkyl then could be in eqm
What are some general features of CO complexes?
CO binds to TM in low oxn states
CO complexes almost always obey 18 VE rule
Used at starting materials
What are the coordination modes of CO complexes?
Binds as terminal C-coordinated 2e- donor
Why is IR used for CO complexes?
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
What is the structure of carbonyl complexes?
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
What increases the chance of M-M bonds?
Stronger down the group as higher d-d overlap
How are TM CO compounds made?
Direct with CO
If 0-valent bulk metals too unreactive, then reduce using Al/CO/H2
What is the σ bonding of CO in TM complexes?
What is the π-bonding of CO in TM complexes?
What do CO ligands prefer to bond to and why?
Prefers to bond to medium to high d e-
π-bonding dominance causes it as e- populate the dπ-CO bonding MOs
Why do CO complexes of main group metals not occur?
Only np AOs can be π-like in bonding but their interaction with the CO 2π (LUMO) is v poor
How does IR of M-CO change across the period?
Increases wavenumber
As Zeff increases wavenumber increases
d-orbital increases in E so better backbonding & weakens CO bond
What is nitrosyl compounds?
NO compounds
What is the MO diagram of nitrosyl compounds?
Free NO has (π*)1 antibonding leading to formal 1e- transfer
How are bridging ligands disruptive of M-M?
Orbitals for M-M used instead for metal-μ-bonding
What is a case when bridging groups disrupt direct element-element bonding?
What are the orbitals on N2?
MOs equally distributed on each N
Gives poorer M-N overlap for both σ-bonding and π-backbonding
How does N2 vs CO2 bond to a metal?
How does N2 bonding change down the group?
Better overlap down the group of metals
More bonding and backbonding, so nitrogen-nitrogen triple bond weakened
