Geometry variation of complex ions in crystals

The geometries of a series of metal complex and other inorganic ions have b een retrieved from crystal structures of their salts in the Cambridge Struc tural Database. These ions show substantially higher variation (sigma) in t heir bond lengths and angles than would be expected on the basis of normal crystallographic estimates of precision (standard uncertainty). There is cl ear 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 [MoC l6](3-) (sigma =0.016 and 0.010 Angstrom respectively) while at the other e xtreme [TeCl6](2-) or [BiCl6](3-) have sigma =0.083 and 0.094 Angstrom resp ectively. Baseline uncertainty levels are apparent in even very well behave d systems such as [PtCl4](2-) and [PtCl6](2-) (bond length and bond angle s igma 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 substant ially 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.