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