1. based on the number of metal atoms present. Depending upon the number of metal atoms present in a metalic carbonyl, they are classified as follows.
(a) Mono nuclear carbonyls. These compounds contain only one metal atom and have simple structures.
For e.g., [Ni (CO)4] Nickel tetra carbonyl is tetrahedral, [Fe(CO)5] Iron pentacarbonyl is trigonalbipyramidal. [Cr(CO)6] Chromium hexacarbonyl is octahedral.
(b) Poly nuclear carbonyls Metallic carbonyls containing two or more metal atoms are called poly nuclear carbonyls.
Poly nuclear metal carbonyls may be –
Homonuclear – ([CO2(CO)6], [Mn2(CO)10], [Fe3(CO)12])
Fletero nuclear – ([MnCO (CO)9], [MnRe (CO)10])
2. based on structure. The structures of binuclear metal carbonyls involve either metal – metal bonds or bridging Co groups or both. The carbonyl ligands that are attached to only one metal atom are referred to as terminal carbonyl groups, whereas those attached to two metal atoms simultaneously are called bridging carbonyls.
Depending upon the structures of metal metal carbonyls they are classified as follows:
(a) Non – bridged metal carbonyls.
Example:- [Ni (CO)4]
These metal carbonyls do not contain any bridging carbonyl lieands. Thev mav be of two tvnes. Nonbridged metal carbonyls which contain terminal carbonyls as well as Metal- Metal bonds.
For examples,The structure of Mn2(CO)10 .
Contains one metal – metal bond, so the formula is more correctly represented as (CO)5Mn – Mn(CO)5 .
(b) Bridged carbonyls: These metal carbonyl contain one or more bridging carbonyl ligands along with terminal carbonyl ligands and one or more metal – metal bonds
For Example: Fe(CO)9 Di-iron ennea carbonyl molecule consists of three CO ligands, six terminal CO groups and single Fe – Fe A bond formed by weak coupling of the unpaired electrons present in two 3d orbitals of 2 Fe atoms. The bond represented by dotted line is called fractional single bond.
