d xy , d yz , d xz XII U5 Coordination XII U5 Coordination - - - - Classification: Generally metal carbonyls are classified in two different ways as described below. (i) Classification based on the number of metal atoms present. Depending upon the number of metal atoms present in a given metallic carbonyl, they are classified as follows. a.
Mononuclear carbonyls These compounds contain only one metal atom, and have comparatively simple structures. For example, Ni CO - Nickel tetracarbonyl is tetrahedral, Fe CO - Iron pentacarbonyl is trigonalbipyramidal, and Cr CO - 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 Co CO Mn CO Fe CO , , or hetero nuclear MnCo CO MnRe CO , etc. (ii) Classification based on the structure: The structures of the 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, metal carbonyls are classified as follows.
a. Non-bridged metal carbonyls: These metal carbonyls do not contain any bridging carbonyl ligands. They may be of two types. (i) Non- bridged metal carbonyls which contain only terminal carbonyls.
Examples: Ni CO , Fe CO and Cr CO CO Ni OC CO CO Fe C C C C C CO OC OC Cr CO CO CO (ii) Non- bridged metal carbonyls which contain terminal carbonyls as well as Metal-Metal bonds. For examples,The structure of Mn (CO) actually involve only a metal–metal bond, so the formula is more correctly represented as (CO) Mn−Mn(CO) . XII U5 Coordination XII U5 Coordination - -