Metal Carbonyls

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This article “Metal Carbolnyls” describes classifications with suitable examples of Carbonyls which helps in easy understanding of Metal Carbonyls. This article covers below point:

  1. What is carbonyls?
  2. Classifications on the basis of
    1. Lgands
    2. ligandsNumber of metal atoms and the structures of Metal Carbonyls

What is metal carbonyls? 
Carbon monoxide is undoubtedly one of the most important and most widely studied ligand in organometallic chemistry. Almost all of the transition metals from metal carbonyls (the complex containing only CO as ligands)and these homoleptic carbonyls are useful precursors for other organometallic compounds.
Though carbon monoxide is not considered as a very strong lewis base, yet it forms a strong bond to the metals in their complexes.
In most of the metal carbonyls, the metals are always in low oxidation state, most often in the oxidation state of zero but sometimes -1 or +1, and these carbonyls are often quite stable with respect to dissociation, substitution or oxidation.CO may bond to a single metal or it may act as a bridge between two or more metals. The number of CO ligands considered to the metal is generally in accordance with the 18-electron rule and this rule is followed by 99% of the metal carbonyls.
Classification of Metal Carbonyls: 

1.Classification on the basis of ligands:

Metal carbonyls can be classified into two categories:

(i)Homoleptic carbonyl complexes: The complex in which the metal is bound to only CO as ligands are known as homoleptic carbonyl complexes.for example, Ni(CO)4, Cr(CO)6,Fe(CO)5,Co2(CO)8,Mn2(CO)10,Fe3(CO)12,Ir4(CO)12 etc.

(ii)Heteroleptic carbonyl complexes: The complexes in which a metal is bound to CO as well as other ligands such as PR3,PPh3,PF3,NO.RNC etc.For example, Ni(CO)3PPh3,Mo(CO)3(PF3)3,Cr(CO)3(NO)2 etc.

2.Classification on the basis of a number of metal atoms and the structures of Metal Carbonyls:

(i)Mononuclear Metal Carbonyls: These carbonyls contain only one metallic atom and these carbonyls do not contain any bridging CO ligand.For example Ni(CO)4,Cr(CO)6,Fe(CO)5 etc.

(ii)Polynuclear Metal Carbonyls: Polynuclear carbonyls contain only one or two metal atoms and these are classified as 

(a)Homonuclear Metal Carbonyls: These contain metal atoms of only one element.

For example : Fe2(CO)9,Mn2(CO)10,Co2(CO)8,Fe3(CO)12,Co4(CO)12,Rh4(CO)12,Ir4(CO)12 etc.

(b)Heteronuclear Metal Carbonyls: These carbonyls contain metals of different elements.

For example: MnCo(CO)4The polynuclear metal carbonyls are also classified as:

(a)Non-bridged Metal Carbonyls: These carbonyls contain terminal CO ligand and M-M bonds.

For example,Co2(CO)8 (In solution),Mn2(CO)10,Ir4(CO)12 etc.

(b)Bridged Metal Carbonyls: These carbonyls contain bridged as well as terminal CO ligands and M-M bonds.For example,Fe2(CO)9,Co2(CO)8 (in solid state),Fe3(CO)12 etc.

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