Organometallics [Andy Johnson] Flashcards
early transition metals - groups 3,4
labile - anion and cation exchange easily
high Zeff
strongly electrophilic and oxophilic
few redox reactions
polar and v. reactive M-C bonds
preference for “hard” σ-donors (non-polarisable)
weak complexation of π-acceptors
typical catalysis = polymerisation
middle transition metals - groups 5-7
ligands bound strongly
strong, not v. reactive M-C bonds
preference for σ-donor/π-acceptor combinations
typical catalysis = alkene and alkyne metathesis
late transition metals - groups 8 - 11
easy ligand association/dissociation
weak - not v. reactive M-C bonds
even weaker/reactive M-O/M-N bonds
preference for s-donor/weak π-acceptor ligands
typical catalysis = hydroformylation
role of catalyst
lowers activation barrier
by introducing a new reaction pathway
** doesn’t change thermodynamics **
why can sigma, pi and delta bonds be treated separately?
they overlap between different classes of orbitals (i.e. net overlap = 0)
criteria for strong orbital interactions
correct symmetry
spatial overlap - must occupy same region in space
similar energy
why can’t pi bonding occur in M-C alkyl bonds?
all available orbitals on C are involved in C-H/C-R bonding
why can’t pi bonding occur in M-C aryl bonds?
it would disrupt the aromaticity of the phenyl ring
what is the most stable state in an 18 e- system?
18 e- involved in bonding orbital
0 e- involved in anti-bonding orbital
L ligand
both electrons provided by ligand
= dative covalent bond
X ligand
one electron provided by ligand and by metal
= normal covalent bond
Z ligand
both electrons provided by metal
= dative covalent bond
metallo
[metal separate from ring]
metal “on” rather than “in”
e.g. ferrocene
metalla
metal “in” ring
oxidation state (in terms of X ligands)
(number of X ligands) + (charge on complex)
bridging ligands
halides often bridge between 2 metal centres using a lone pair
= LX
X = sigma bond
L = donor bond
metal-metal bonds
[doesn’t add to oxidation state]
assign one of the electrons of the electron pair in a single to each metal centre
double, triple, quadruple -> 2,3,4 respectively
NO molecules
radical
BENT - acts as a 1 electron X ligand (l.p. on nitrogen not used)
LINEAR - l.p. now can be used (LX)
electronic effects
late TM d8 -> square planar
d10 -> trigonal
-as Z increases, d-shell is stabilised
-occupied dx2 orbital (perpendicular to plane) no longer involved in ligand bonding
steric effects
early TM have less d-electrons
-> must achieve 18 e- count via larger ligands
if ligands = too bulky, low-electron count complexes are formed
back-bonding
ligand donates sigma/non-bonding e- to metal while accepting e- density through overlap of metal t2g orbital and ligand π*
^ ligand = both sigma donor and π acceptor
what happens as the amount of pi e- density donated into pi* ligand increases?
more backhanding/electron density in ligand anti bonding orbital
weakens CO bond
bond lengthens
lowers CO stretching frequency
why is there an increase in CO frequency/bond order across the periodic table group?
d orbital energies decrease/increase in electron density on M
energies of dπ and π* orbitals separate
back bonding becomes worse
non-classical carbonyls
major interaction = σ-donation from CO 5σ orbital (anti-bonding, l.p. on CO) to metal
stretching frequency > free CO
relationship between wave number and bond length
wave number is proportional to 1/bond length
how many electrons does CO contribute?
2e- whether terminal or bridging
how can we fix a molecule which is coordinatively unsaturated?
M-M bonds
what type of ligand are phosphine ligands?
L type
2 important parameters of PR3 ligands
[electronic] - donor and acceptor strength
-> change R groups
[sterics]
size of sub. -> Tolman Cone Angle
what do phosphine ligands primarily function as?
Lewis bases
interact with metals as σ donor ligands
phosphine ligands - electronics
e- density on P is low when R group contains EWG
e- density on P is high when R group contains EDG
why can’t NR3 participate in back-bonding?
strong σ-donors BUT have no accessible molecular orbitals available with correct symmetry
PR3, AsR3, SbR3
descending 15 σ* orbitals which bond R groups to P, As and Sb -> get lower in energy
makes it more available
why does M-P bond lengths increase and P-R bond lengths decrease upon oxidation?
oxidation decreases ability of metal to backbone -> removes e- density from metal
=> would cause M-P bond length to increase
σ*P-R orbitals are used in back-bonding -> stops elongating with less back-bonding
CO stretching frequency trend
more e- density on metal
= more back bonding onto CO
= greater occupancy of π* CO
= more electron density on M
= lower CO stretching frequency
what does wider cone angles indicate?
greater steric congestion
= more labile phosphine ligand
how does V(CO) change as the electron-donating ability of the phosphine ligand increases?
strongly donating = more e- rich metal
better at CO back-bonding
= lower V(CO) due to decreased C-O bond order
C-C π to empty metal orbital (σ)
e- density moves from bonding orbital to empty metal orbital
depopulation of π orbital = C-C bond length increasing
M-C = SHORTER
occupied metal d -> empty C-C π*
e- density moves from full M orbital to anti-bonding C-C orbital
population of π* orbital leads to C-C length increases
M-C = SHORTER
factors which favour π-back bonding
[make M e- rich]
strong donor ligands on M
negative charge on M
low oxn state on M
how does back bonding affect alkene reactivity?
back bonding reduces δ+ charge
reduces reactivity to nucleophiles
how do compounds relieve strain?
hybridisation
sp2 -> sp3 = less strain
metal alkynes
more electronegative
encourage back bonding
bind more strongly to metals
activation
cleavage of organic bonds by metal centres
produces more reactive form of organic species
oxidative addition - favoured conditions
favoured with more electron rich metal centres
-> low oxidation states
-> negative charge
-> ED CO-ligands
PPh3 vs P(OPh)3
[PPh3]
- stronger σ donor
- more e- density on metal
- strong Co-H
[P(OPh)3]
- better π-acceptor (electronegative group)
- more back bonding
-less e- density on metal
- Co-H = weaker and more acidic
M-C bond enthalpy
decreases with atomic number
increases within a TM triad
sigma bond metathesis compared to oxidative addition/reductive elimination
sigma bond metathesis -> no change in oxidation state
oxidative addition/reductive elimination -> 2 electron change in oxidation state
problems for C-H activation - THERMODYNAMICS
ΔS - disfavoured entropically (2 -> 1 molecules)
ΔH - must be sufficiently exothermic to overcome loss of entropy
H-H vs CH3-H
H-H and CH3-H have similar strengths
-> weaker M-alkyl bonds disfavour C(sp3)-H activation
Ph-H vs CH3-H
energy required to break the stronger Ph-H is outweighed by strong M-Ph and M-H bonds in the product
-> Ph-H bond activation much easier than sp3-C-H activation
what is a negative ΔS of activation indicative of?
associative mechanism
what is a positive ΔS of activation indicative of?
dissociative mechanism
what are Fischer carbenes direct analogues of?
reactivity of carboxylic acids
both have electrophilic carbon centres with leaving groups
sterically crowded early TM alkyls
undergo alpha-elimination readily to give nucleophilic carbenes and alkanes
what promotes carbene formation?
increasing steric strain in metal complex
