COORDINATION COMPOUND (Metal Carbonyl)

Metal Carbonyl( for classXII)

Dear students,
                           Let us discuss today about metal(transition metals) carbonyl.
Transition metal atoms have ability to form complexes with a variety of neutral
molecules (for e.g carbon monoxide,nitric oxide,arsine) and various molecules
with delocalized pi orbitals such  as Benzene.
In many complexes ,the metal atoms are in low positive, zero,even negative
formal oxidation states.Few ligands can stabilize low oxidation state as they

have vacant pi orbitals in addition to lone pairs.These vacant orbitals accept 

electron density from filled metal orbitals to form a type of pi bonding that
supplements the sigma bonding arising from lone pair donation.High electron
density on the metal atom (low oxidation state) can thus be delocalized into
the ligands.The ability of ligands to accept electron density into low lying
empty pi orbitals is called pi acidity(pi acceptor).
The first metal carbonyl Ni(CO)₄.was discovered by A.Mond.An inert
molecule like CO  is uniting to form stable compound,especially when
the lewis basicity of CO is very less?  However,the explanation lies in the
multiple nature of the metal-carbonyl bond.

• A dative overlap of  the filled carbon sigma orbital.
• A dative overlap of a filled d-pi metal orbital with an
•  empty antibonding p-pi orbital of the CO

The drift of electrons to the metal in the sigma tends to make the CO as a whole
electron rich, which will increase its basicity .Secondly the sigma orbital of carbon
will drift electrons to the metal.The sigma bond tends to make the CO positive,
thus enhancing the acceptor strength of the antibonding pi orbitals.
         This bonding mechanism is synergic as  sigma bond formation strengthen
the pi bonding and vice versa.The extent of  back-donation from M to CO

increases,the M-C bond become stronger and the CO triple bond become

weaker. Thus the mutiple bonding should be evidenced by shorter M-C bond

and longer CO bonds as compared with M-C single bonds and CO triple bonds.