No abstract

Some 60 examples of crystal structures are presented which can be better described in space groups of higher symmetry than used in the original publications. These are divided into three categories: (A) incorrect Laue group (33 examples), (B) omission of a center of symmetry (22 examples), (C) omission of a center of symmetry coupled with a failure to recognize systematic absences (nine examples). Category A errors do not lead to significant errors in molecular geometry, but these do accompany the two other types of error. There are 19 of the current set of examples which have publication dates of 1996 or later. Critical scrutiny on the part of authors, editors and referees is needed to eliminate such errors in order not to impair the role of crystal structure analysis as the chemical court of last resort

Due to the paucity of data on non-bonding interactions for metal atoms, no complete tabulation is available for crystallographic van der Waals radii for metallic elements. In this work several sets of van der Waals radii for metal atoms are derived indirectly. Unique data resources used for the derivation are (i) average volumes of elements in crystals, (ii) single covalent radii, (iii) Allinger´s van der Waals radii, as well as (iv) bond valence parameters for metal-oxygen bonds. The van der Waals radii for metal atoms deduced from these various approaches are basically comparable with each other, but are strikingly different from those from Bondi’s system of van der Waals radii. A complete set of new values for metallic elements up to Am, derived from bond valence parameters, are recommended.

No abstract

Important data sets of crystallographic van der Waals radii covering major portions of the Periodic Table are reviewed, including those originally published in Pauling's pioneering work in the late 1930s, together with other more recent data sets. The van der Waals radii for rare gases are determined next from short contact distances in crystallographic data, as an illustration of the value of crystal structure databases for the direct determination of van der Waals radii for non-metallic elements. Then, van der Waals radii for the other non-metallic and metallic elements, as presented in the literature and having been determined using various approaches, are compared systematically. Finally, certain outstanding problems related to the derivation and application of van der Waals radii are discussed, and further investigations of crystallographic van der Waals radii are suggested.