Geometry variation of complex ions in crystals

The geometries of a series of metal complex and other inorganic ions have been retrieved from crystal structures of their salts in the Cambridge Structural Database. These ions show substantially higher variation (sigma) in their bond lengths and angles than would be expected on the basis of normal crystallographic estimates of precision (standard uncertainty). There is clear evidence of a link between sigma and the softness of systems. Thus M-Cl bond length sigma values are small for species such as [SnCl6](2-) or [MoCl6](3-) (sigma =0.016 and 0.010 Angstrom respectively) while at the other extreme [TeCl6](2-) or [BiCl6](3-) have sigma =0.083 and 0.094 Angstrom respectively. Baseline uncertainty levels are apparent in even very well behaved systems such as [PtCl4](2-) and [PtCl6](2-) (bond length and bond angle sigma values of ca. 0.01 Angstrom and 1.0 degrees respectively). There is no evidence that the geometries of the complex ions studied here are substantially more variable than those of similar neutral molecules. There is some evidence that the organic or non-metal aspects of molecular geometry are less variable than the metal-containing aspects.