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At the opposite extreme, steric shielding can give rise to unusually low coordination numbers. Among the Frank–Kasper phases, the packing of metallic atoms can give coordination numbers of up to 16. One computational chemistry study predicted a particularly stable PbHe 2+ġ5 ion composed of a central lead ion coordinated with no fewer than 15 helium atoms. When the surrounding ligands are much smaller than the central atom, even higher coordination numbers may be possible. For example, with bidentate nitrate ions as ligands, Ce IV and Th IV form the 12-coordinate ions 2− ( ceric ammonium nitrate) and 2−. Coordination numbers of 8 to 12 are commonly observed for f-block elements. Metals in the f-block (the lanthanoids and actinoids) can accommodate higher coordination number due to their greater ionic radii and availability of more orbitals for bonding. octahedral vs trigonal prismatic.įor transition metal complexes, coordination numbers range from 2 (e.g., Au I in Ph 3PAuCl) to 9 (e.g., Re VII in 2−). The CN does not distinguish the geometry of such complexes, i.e. The most common coordination number for d-block transition metal complexes is 6. The concept is most commonly applied to coordination complexes. In chemistry, coordination number (C.N.), defined originally in 1893 by Alfred Werner, is the total number of neighbors of a central atom in a molecule or ion. Chloro(triphenylphosphine)gold(I), which features 2-coordinate metal centre.