Metal carbonyl compounds And Bonding in metal carbonyl

Metal carbonyl compounds-

  ·       are organometallic compounds which have at least one carbon-metal bond.

Examples-

[Ni(CO)₄ ] Tetracarbonylnickel(0)

[Fe(CO)₅] Pentacarbonyliron(0)

[Cr(CO)₆] Hexacarborbonylcromium(0)

[Fe₂(CO)₉]  Tri-µ-carbonyl bis(tricarbonyl iron)

[Mn₂(CO)₁₀]   Decacarbonyldimanganese

((CO)₃ – Co – (CO)₂ – Co – (CO)₃ Di-µ-carbonylbis(tricarbonylcobalt)

[M(CO)₆] [M = Cr, Mo, W]

Bonding in metal carbonyl-

Metal -carbon bond in metal carbonyls possess both σ and π character.the formation of bonds between the metal and carbon atom of monoxide can be explained as-

·       The first overlap take place between the filled bonding π_2p orbital of the carbon monoxide with an empty metal d-orbitals resulting in a σ-bond between the metal and carbon atom of carbon monoxide. here, donation of lone pair of electrons on carbon into a vacant d-orbital of metal takes place.

·       The second overlap(is also called Pi-back bonding) takes place between the filled d-orbital of metal with an empty antibonding orbital π^*_2p of  the carbon monoxide resulting in π-bond between the metal and the same carbon monoxide molecule. here, donation of lone pair electrons  on a filled d-orbital into empty antibonding orbital π^*_2p of the carbon monoxide.

·       In other words, the M-C σ-bond is formed by donation of lone pair of electrons on the carbonyl carbon into vacant d-orbtal of the metal. The M-C π-bond formed by the donation of lone pair electrons from a filled d-orbital of the metal into vacant antibonding orbital π^*_2p of the carbon monoxide(called back bonding or Pi-back bonding). 

·       Pi-back-bonding is also known as Synergic bonding which means self-strengthening bond.

·       Synergic effect-the effect of σ-bond formation strengthens the π-bond and vice versa. It is called synergic effect.