L7/8

Front 1

What two types of complexes do organic molecules form with transition metals ?

Back 1

1. Those containing metal-carbon sigma bondse.g. carbonyls, alkyls, carbenes.

2.Those interacting with the metal via their pi-electrons.e.g. alkenes, arenes.

Front 2

What type of bonding is unique to transition metals ?

Back 2

Organic molecules that interacting with metals via their p-electrons. e.g. alkenes,arenes. This type of bonding is unique to transition metals. Metals whoseenergetically accessible d-orbitals allow efficient overlap with the p-orbitals of carbon-carbonmultiple bonds to give "p-complexes".

Front 3

Draw the preferred orientation of an alkene bonded to a metal via square planar geometry and explain why ?

Back 3

Draw A from L7/8.

The orientation of the alkene is due to steric factors. There is less steric interaction with other ligands when the alkene isperpendicular to the square planar.

Front 4

What are the 3 ways to synthesize alkene-metal complexes ?

Back 4

1.Ligand Displacement:CO can be displaced from iron carbonyl by the alkene under forcingconditions.

2.Protonation of a h1-allyl species:

3.Beta-Hydride Abstraction of an Alkyl Ligand:

Front 5

Describe how alkenes bond to metals ?

Back 5

Alkenes bind to transition metals by donation of the C=C pi electrons to an empty ds orbital on the metal. However,since alkenes are weak bases, a s-bondis insufficient on its own to stabilise the complex. So by analogy with thecarbonyl ligand this is accompanied by back donation from an occupied metal dp orbital into the ligandLUMO, the C=C pi*level. The metal therefore acts as both a Lewis acid (electron acceptor) and aLewis base (electron donor) with respect to the alkene. This bonding conceptwas first proposed by Dewar and by Chatt and Duncanson.So only metals capableof back-donation bind alkenes and not d0 metals such as Ti(IV).n

Front 6

What are the structural consequences of back-bonding intothe alkene pi* levels ?

Back 6

1. Lengthening of the C=C bond.

2. Reduction of the angles around C from 120° (sp2) towardsangles typical of tetrahedral (sp3-C).

Front 7

What factors effect the extent of back bonding ?

Back 7

1. Steric effects

2. Electron-acceptor strength of the olefin, i.e. thepresence/absence of electron withdrawing groups.

Front 8

How do alkenes with electron withdrawing substituents effect back bonding ?

Back 8

The degree of back-bonding is dependent on the acceptorcapacity of the alkene. This is strongly enhanced by electron-withdrawingsubstituents. F and CN groups, in particular, render the alkene stronglyelectrophilic so that the resulting metal complexes generally adopt ametallacyclopropane structure, with almost tetrahedral carbon atoms.

Front 9

How does a zero valence metal effect back bonding, draw an example ?

Back 9

Much more extensive back bonding can be expected fromzero-valent metals. This complex is in agreement with metallacyclopropanedescription and the C-C bond is very long, 1.46Å.