ofofofofof Ni OC Ni(CO) Tetrahedral Fe OC OC Fe(CO) Trigonal bipyramidal Cr OC CO CO Mn Mn Cr(CO) Octahedral [Mn (CO) ] Co Co OC OC [Co (CO) ] OC C O O C Fig. . Structures of some representative homoleptic metal carbonyls. The metal-carbon bond in metal carbonyls possess both s and p character.
The M–C s bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal. The M–C p bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding p * orbital of carbon monoxide. The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal (Fig. .
). Fig. . : Example of synergic bonding interactions in a carbonyl complex.
The coordination compounds are of great importance. These compounds are widely present in the mineral, plant and animal worlds and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. These are described below: Coordination compounds find use in many qualitative and quantitative chemical analysis. The familiar colour reactions given by metal ions with a number of ligands (especially chelating ligands), as a result of formation of coordination entities, form the basis for their detection and estimation by classical and instrumental methods of analysis.
Examples of such reagents include EDTA, DMG (dimethylglyoxime), a –nitroso– b –naphthol, cupron, etc. Hardness of water is estimated by simple titration with Na EDTA. The Ca + and Mg + ions form stable complexes with EDTA. The selective estimation of these ions can be done due to difference in the stability constants of calcium and magnesium complexes.
Some important extraction processes of metals, like those of silver and gold, make use of complex formation. Gold, for example, combines with cyanide in the presence of oxygen and water to form the coordination entity [Au(CN) ] – in aqueous solution. Gold can be separated in metallic form from this solution by the addition of zinc. Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds.