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In chemistry, crystallography, and materials science the coordination number of a central atom in a molecule or crystal is the number of its near neighbours. This number is determined somewhat differently for molecules than for crystals. For molecules and polyatomic ions the coordination number of an atom is determined by simply counting the other atoms to which it is bonded (by either single or multiple bonds). For example, ()1− has Cr3+ as its central cation, and has a coordination number of 6. However the solid-state structures of crystals often have less clearly defined bonds, and in these cases a count of neighbouring atoms is employed. The simplest method is one used in materials science. The usual value of the coordination number for a given structure refers to an atom in the interior of a crystal lattice with neighbours in all directions. In contexts where crystal surfaces are important, such as materials science and heterogeneous catalysis, the value for an interior atom is the bulk coordination number, while the value for an atom at a surface of the crystal is the surface coordination number. == Molecules, polyatomic ions and coordination complexes == 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.〔〔A.K. De (2003) ''A Text Book of Inorganic Chemistry'', p. 88. New Age International Publishers, ISBN 8122413846〕 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 (HC≡CH). In effect we count the first bond (or sigma bond) to each neighbouring atom, but not the other bonds (pi bonds). In coordination complexes, 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. For π-electron ligands such as the cyclopentadienide ion ()−, alkenes and the cyclooctatetraenide ion ()2−, the number of atoms in the π-electron system that bind to the central atom is termed the hapticity.〔>IUPAC ''Gold Book'' definition: (''hapticity'' )〕 In ferrocene the hapticity, η, of each cyclopentadienide anion is five, Fe(η5-C5H5)2. There are various ways of assigning the contribution made to the coordination number of the central iron atom by each cyclopentadienide ligand. The contribution could be assigned as one since there is one ligand, or as five since there are five neighbouring atoms, or as three since there are three electron pairs involved. Normally the count of electron pairs is taken.〔The Organometallic Chemistry of the Transition Metals, Robert H. Crabtree, (2009), John Wiley & Sons, ISBN 978-0-470-25762-3〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Coordination number」の詳細全文を読む スポンサード リンク
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