The rule states that thermodynamically stable transition metal organometalliccompounds are formed when the sum of the metal d electrons and the electrons conventionally considered as being supplied by the surrounding ligands equals 18.
In general, the conditions favoring adherence to the 18 electron rule are, an electron rich metal (one that is in a low oxidation state) and ligands that are good π‐acceptors
The hapto symbol, η,with a numerical superscript, provides a topological description by indicating the connectivity between the ligand and the central atom. For example, if all the five carbon atoms of a cyclopentadienyl moiety are equidistant from a metal atom, we term it as
η5‐cyclopentadienyl
Examples: η1‐R, η1‐Ar, η2‐C2R4, η1‐allyl, η3‐allyl, η4‐Cb, η5‐Cp, η6‐C6H6, η8‐C8H8, η2‐C60, η5‐ R5C60.
The symbolμindicates bridging; normally we have
μ1 and rarely μ3bridging
Examples: μ2‐CO, μ3‐CO, μ2‐CH3, μ2‐H, μ2‐Cl, μ3‐Cl, μ2‐OR, μ2‐PR2, μ2‐NR2
Methods Of Counting:
Neutral Atom Method & Oxidation State Method
Neutral atom method: Metal is taken as in zero oxidation state for counting purpose
Oxidation state method: We first arrive at the oxidation state of the metal by considering the number of anionic ligandspresent and overall charge of the complex
[Suggestion: Focus on one counting method till you are confident]
Easy Way to Remember ligand electron contribution for NEUTRAL ATOM COUNTING method:
APPLICATIONS OF 18 Electron Rule
How to determine the total number of metal ‐metal bonds
Determine the total valence electrons (TVE) in the entire molecule (that is, the number of valence electrons of the metal plus the number of electrons from each ligand and the charge); say, it is A.
Subtract this number from n×18 where n is the number of metals in the complex, that is, (n×18) –A; say, it is B.
(a) B divided by 2 gives the total number of M–M bonds in the complex.
(b) A divided by n gives the number of electrons per metal. If the number of electrons is 18, it indicates that there is no M–M bond;
if it is 17 electrons, it indicates that there is 1 M–M bond;
if it is 16 electrons, it indicates that there are 2 M–M bonds and so on.
Few More Examples:
Exceptions to the 18 electron rule
• Square planar organometallic complexes of the late transition metals (16e).• Some organometallic complexes of the early transition metals (e.g.Cp2 TiCl2, WMe 6, Me 2NbCl3 ,CpWOCl3) [A possible reason for the same is that some of the orbitals of these complexes are too high in energy for effective utilization in bonding or the ligands are mostly σ donors.]
• Some high valent d0 complexes have a lower electron count than 18.
• Sterically demanding bulky ligands force complexes to have less than 18 electrons.
• The 18 electron rule fails when bonding of organometallic clusters of moderate to big sizes (6 Metal atoms and above) are considered.
• The rule is not applicable to organometallic compounds of main group metals as well as to those of lanthanide and actinide metals.
Why We Study Metal Carbonyls????
A range of metal carbonyls are used as catalysts in Chemical Industry
:BONUS:
Molecular Orbital diagram of CO
Why does CO bind a metal through its less electronegative carbon atom than its more electronegative oxygen ? What makes it a good π acceptor
The highest occupied molecular orbital (HOMO) of CO is weakly antibonding (compared with the O atomic orbitals) and is an MO which is carbon based. Secondly, the π* antibonding orbital which is the lowest unoccupied molecular orbital (LUMO) is also of comparatively lower energy which makes it possible to interact with metal t2g orbitals for πbonding. There exists a strong back bonding of metal electrons to the π* anti bonding orbitals of CO
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Problems Asked in CSIR and Other Examinations:
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