text: The most common coordination number for d-block transition metal complexes is 6.  The coordination number does not distinguish the geometry of such complexes, i.e. octahedral vs trigonal prismatic.  
  For transition metal complexes, coordination numbers range from 2 (e.g., Au in PhPAuCl) to 9 (e.g., Re in [ReH]).  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.  Coordination numbers of 8 to 12 are commonly observed for f-block elements.  For example, with bidentate nitrate ions as ligands, Ce and Th form the 12-coordinate ions [Ce(NO)] (ceric ammonium nitrate) and [Th(NO)].  When the surrounding ligands are much smaller than the central atom, even higher coordination numbers may be possible. One computational chemistry study predicted a particularly stable  ion composed of a central lead ion coordinated with no fewer than 15 helium atoms. Among the Frank–Kasper phases, the packing of metallic atoms can give coordination numbers of up to 16. At the opposite extreme, steric shielding can give rise to unusually low coordination numbers.  An extremely rare instance of a metal adopting a coordination number of 1 occurs in the terphenyl-based arylthallium(I) complex 2,6-TippCHTl, where Tipp is the 2,4,6-triisopropylphenyl group.