chem313-311-lecture 9-2011
TRANSCRIPT
Metal-cyclopentadienylcomplexes
CHEM311/313
Conjugated dienes - bonding
In conjugated dienes/polyenes the bonding of the entire conjugated unit must be considered “as one”. the C=C bonds cannot be
treated as separate units.
Donation can occur from the diene HOMO to an
empty metal orbital from a full d-orbital to the LUMO
of the diene
B2 p 15-17
Donation from the HOMO
• Strong -donation from 2to the metal
• depopulates C1=C2 bond • weaker, longer
• depopulates C2=C3 anti-bond• stronger, shorter
• all bonds same length
• di-ene type structures with weakened C=C•
B2 p 16
C-C anti-bonding
Back-donation into the LUMO
electrons flow from metal dxz-orbital into 3 C1-C2 anti-bonding C2-C3 bonding C3-C4 anti-bonding
bond lengths long-short-long metallacycle
B2 p 16
M
C-C bonding
Aromatic polyolefins
If the olefin is aromatic then the -cloud can co-ordinate.
The no. of electrons donated = the number of -electrons
The ligand occupies (no. of electrons donated)/2 co-ordination sites
Cyclopentadienide
C5H5_
(Cp) has 6 -electrons ferrocene is its archetypical complex
Large family of metallocene complexes
vs2+
Synthesis of cyclopentadienyl compounds
Naheat2
Co2+
2 - Na Fe2+Fe
MCl4
MCl
Cl
Co
MO picture of the bonding
a1 ligand orbital has correct symmetry to interact with dz2 but lie on the dz2
nodal plane, i.e. are in the “wrong” place.
e1g ligand set overlaps with the metal dxz and dyz orbitals forming a strong set of -bonds.
e2g ligand orbitals can overlap with the metal dx2-y2 and dxy but the degree of overlap is not large so these MOs are essentially non-bonding.
e1u interaction between the metal pxand py also gives some stabilization.
B1 p. 46-47 http://www.ilpi.com/organomet/cp.html
Simplified MOs for eclipsed conformation
http://www.nuigalway.ie/cryst/oscail_tutorial/moilin/ferrocene/ferrocene.htm but watch out for the typos!
a1g overlap weak e1g overlap strong
MO diagram
Fe(II) d6 ion 6 metal d electrons
12 electrons donated by the 2 Cp ligands
Ferrocene and 18 electron rule
Lowest MOs are the 6 ligand to metal bonding orbitals – ligand based
There are 3 weakly bonding, or non-bonding metal based orbitals next in energy
18 electrons are needed to fill all 9 orbitals above
BUT, no strong NEED to fill the non-bonding MOs e.g. ferrocene is readily oxidized to [(Cp)2Fe]+
Sandwich complexes with 15 to 20 valence electrons are known (vide infra)
Metallocenes & redox
electrons are easily removed from/added to the HOMO/LUMO
metallocenes are mild oxidizing/reducing agents
can be used as electron transfer catalysts ExacTech pen meter
Sandwich compounds
metal based HOMOs & LUMO are essentially non-bonding
no NEED to fill HOMOs small penalty on filling the LUMO 6 L M bonding orbitals must be filled (12
electrons) total electron counts from 15 – 20 are
known
B2 p 476 L M bonding
MOs
ExacTech pen meter
B2 p 51, H&S p 731
Reaction at the ring or metal
H&S p 732
Ring whizzer
staggered and eclipsed conformations have very similar energies and MOs.
1H and 13C{1H} NMR spectra usually show a single resonance even at low T, even if the metal environment makes the ring carbons inequivalent
Ring whizzer
a1 orbital contributes a small “non-directional” component to the bonding
C5H5 fragment can spin like a plate on a stick
Unlikely to be explanation, since the e1goverlaps are strongly bonding
Ring whizzer
Unlikely to be explanation, since the e1g overlaps are strongly bonding
Overlaps are maintained as conformation changes from staggered to eclipsed.
Small barrier to overlaps “hopping around the ring
C5H5 fragment can spin like a plate on a stick
http://alpha.chem.umb.edu/chemistry/ch612/documents/ConstructingpiMOdiagramssandwichcomplexes.pdf
Rare example of inequivalence in the NMR spectra
steric hinderance between the phenyl group and the Cp protons allows the ring whizzing to be frozen out at -100 oC
separate resonances are observed for each carbon atom of Cp
H
HOs
PPh3PPh3
Ph
Other bonding modes for cyclopentadiene
cyclopentadiene can also behave as an alkyl ligand
or as an 3 ligand
Ring slippage changes electron count at metal loss of aromaticity
no reaction
redistribution of aromaticity reaction goes
B2 p 61
Many metal-arene complexes known
Benzene and its derivatives have similar frontier orbitalsand same number of valence electrons as Cp-
Similar complexes are known
Similar redox and magnetic properties
Similar reactivity
B2 p 76
Synthesis
B2 p 75
cf Reactivity at the ring in arenes
reactivity mirrors that of Cp- in ferrocene
B2 p 79