Chemistry Usage
In chemistry, coordination number (c.n.), defined originally in 1893 by Alfred Werner, is the total number of neighbours of a central atom in a molecule or ion. Although a carbon atom has four chemical bonds in most stable molecules, the coordination number of each carbon is four in methane (CH4), three in ethylene (H2C=CH2, each C is bonded to 2H + 1C = 3 atoms), and two in acetylene. In effect we count the first bond (or sigma bond) to each neighbouring atom, but not the other bonds (pi bonds).
In inorganic chemistry also, only the first or sigma bond between each ligand and the central atom counts, but not any pi bonds to the same ligands. In tungsten hexacarbonyl, W(CO)6, the coordination number of tungsten (W) is counted as six although pi as well as sigma bonding is important in such metal carbonyls.
Examples of high coordination number complexes are the ions formed by uranium and thorium with bidentate nitrate ion ligands, U(NO3)62− and Th(NO3)62−. Here each nitrate ligand is bound to the metal by two oxygen atoms, so that the total coordination number of the U or Th atom is 12.
When the surrounding ligands are smaller than the central atom, even higher coordination numbers may be possible. One computational chemistry study predicted a particularly stable PbHe152+ ion composed of a central lead ion coordinated with no fewer than 15 helium atoms.
Read more about this topic: Coordination Number
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