Definition of an organometallic compound
Systematic Nomenclature
18-Electron Rule

Definition of an organometallic compound

A compound that contains a metal-carbon bond. A metal is seen as any element less electronegative than carbon (this includes B, Si and As - and excludes P and halogen) top

Nomenclature of Organometallic Compounds

The nomenclature for organometallics involves elements from classical organic and inorganic nomenclature with some extra rules. These rules mainly explain the structure of the compound and can indicate how the metal is bonded to the organic parts.

  1. Put the organic groups in ALPHABETICAL order BEFORE the metal.

    2. (Use di,tri or bis,tris as appropriate)
    .
  2. Anionic groups can
      3. either : take ending -o + prefix metal
      or : take ending -ide + suffix metal
      Some examples
  3. If species is anionic the metal takes ending -ate + O.S.

    4. An example
    .
  4. A positive (cationic) or neutral species keeps element name + O.S.

    5. An example
    .
  5. Groups IV and V, the "metal" can be named after the metal hydride.

    6. Some examples
    .
  6. -bonded systems

    7. With double bonded, conjugated and ring systems -bonds can occur with the metal. To name these systems use HAPTICITY. Hapticity means the number of carbon atoms bonded to the metal. The symbol for this is the "eta": .

    <== indicates number of carbons bonded to the metal.
    Some examples
    .

  7. Bridged systems

    8. Here CO is acting as both a terminal and a bridging ligand. To indicate this in the name use the symbol "mu": µ.

    <== indicates number of (metal)atoms being bridged.
    Some examples

NB: You can use nomenclature at different levels for different purposes. At one end, eg: 
      - just to order a chemical 
      - use simplest name

      Fe2(CO)9 ==> nonacarbonyldiiron

Or at the other extreme to make subtle points about structure usefull description:
      Fe2(CO)9 ==> Tri-µ2-carbonylbis(tricarbonyliron)

After you have studied the previous theory, you can practice some problems. top

The 18-Electron Rule

Most -complexes of the transition metals conform to an emperical rule, know as the "The Eighteen Electron Rule ", which assumes that the metal atom accepts sufficient electrons from the ligands to enable it to attain the electronic configuration of the next noble gas. This means that the valence shell of the metal atom contains 18 electrons. Thus, the sum of the of the number of metal d-electrons plus the electrons conventionally regarded as being supplied by the ligands is 18. This leads to a constant electronic arrangement for the metal atom.
(Reference: 3)

Procedure for counting electrons

  1. Count number of electrons (e-) in valence shell of free metal atom.

    2. Fe0 ==> 8 (e-) (4s2 3d6 or just count the number of groups in the "periodic system" example).

  2. Add or substratct electrons depending on the total charge on the metal complex.

    3. [Fe(L)x] 0 ==> 8 (e-)
    [Fe(L)y] +==> 7 (e-)
    [Fe(L)z] - ==> 9 (e-)

  3. Sum the numbers of electrons that the ligands formally contribute to the metal.

    4. terminal CO ==> 2 (e-)
    bridging CO ==> 1 (e-)
    metal-metal bond ==> 1 (e-)
    organic ligands ==> number of electons depends on the group: see table

  4. Sum the electrons find in 2 and 3. A stable complex will have 18 electrons.

Some examples of the Eighteen Electron Rule
(Click on the structures to see the ligand classification table)

Fe(CO)5:

Feo:                      8e-
5xCOT:              10e-
Total:                 18e-

Fe2(CO)9:
In this case the molecule is split in half because of the two iron atoms which both want to attain the electronic configuration of the next noble gas.

Feo:                 8e-
3xCOT:           6e-
3xCOB:           3e-
Fe-Fe bond:    1e-
Total:             18e-

 

In order to achieve stabillity elements with less than 18 electrons in their valence shell have to dimerise or either accept an electron to form an anion.

eg: Mn(CO)5

Mno:          7e-
5xCOT:    10e-
Total:        17e-

This molecule is unstable, here are the two ways to fill the valence shell of the mangenese:

Dimerisation:

Mno:                 7e-
5xCOT:           10e-
Mn-Mn bond:    1e-
Total:               18e-

Accepting an electron:

Mn1-:         8e-
5xCOT:   10e-
Total:       18e-

 

Predicting structures using the Eighteen Electron Rule

The Eighteen Electron Rule will indicate stoichiometries and possible geometries of compounds. This rule will not differentiate between possible structures, to determen the actual structure of a compound techniques like IR or NMR are necessary.
Here's an example:

Bis[tetracarbonylcobalt] can exist in two forms:

Structure A: Coo:                 9e-
4xCOT:            8e-
Co-Co bond:    1e-
Total:              18e-
Structure B: Coo:                9e-
3xCOT:           6e-
2xCOB:           2e-
Co-Co bond:   1e-
Total:             18e-

Structure A as well as structure B are possible. From NMR spectra it appears that structure A is the solid state and low temperature-solution state and structure B is the high temperature-solution form.


Exceptions

To fill the outer shell with 18 electrons there must be 18 electrons available for bonding.

  1. This is not the case for metals at the ends of the transition series: Sc, Ti, Ni, Cu. Particularly if the element is prone to form square-planar complexes, this will result in 16 electrons.
  2. Steric limitations can also prevent the metal from having 18 electrons in the outer shell.
  3. Metallocenes ( like: M(5-C5H5)2 ) form constant complexes with metals, in the periodic table from the tin group until the nickel group. Only the iron complex has 18 electrons but the other complexes are also more stable than would be predicted for their number of electrons.

After you have studied the previous theory, you can practice some problems. top