sentences:  One computational chemistry study predicted a particularly stable ion composed of a central lead ion coordinated with no fewer than 15 helium atoms.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-Tipp2C6H3Tl, where Tipp is the 2,4,6-triisopropylphenyl group.