Chem. Mater. 2007, 19, 1199
1199
Comments Comment on “Distortions in Octahedrally Coordinated d0 Transition Metal Oxides: A Continuous Symmetry Measures Approach” In a recent paper,1 statistics of d0 cation displacements in oxygen octahedra was analyzed, and several conclusions were drawn. Some of them, like division into strong, moderate, and weak distorters, have been known for decades2,3 (still one of strong distorters, Re(7+),3 was missed by Ok et al.1). Some other conclusions seem doubtful because they are based on a limited amount of data, about 5-25% of relevant structures available from the ICSD,4 and some important cases or even classes of compounds might be missed. Just one example of an incorrect conclusion is as follows. The authors report that no vertex-directed distortions are observed for Mo(6+). However, in numerous Keggin structures, typified by various 12-molybdophosphoric acids5,6 and their
salts, each of the 12 Mo(6+) ions has a pseudotetragonal oxygen environment of 1 + 4 + 1. Similar bias may be suspected with other analyzed d0 cations, and, thus, the statistical data reported in Tables 1 and 2 do not seem reliable. In addition, the authors do not explain what constitutes an example: a phase, an ICSD entry, or an individual octahedron. For example, the abovementioned triclinic acid5 has 12 nonequivalent MoO6 octahedra, whereas in the cubic acid,6 all 12 Mo atoms are identical. Then, how many examples of MoO6 octahedra are there: 2, 13, or 24? The authors should have indicated how they selected structures for analysis, why not all relevant structures were used, and how they counted polyhedra in structures with several symmetry nonequivalent octahedra. To obtain reliable statistics, it is necessary to survey the entire ICSD. A good example of such careful analysis was reported by Brown.7
V. B. Nalbandyan (1) Ok, K. M.; Halasyamani, P. S.; Casanova, D.; Llunell, M.; Alemany, P.; Alvarez, S. Chem. Mater. 2006, 18, 3176-3183. (2) Porai-Koshits, M. A.; Atovmyan, L. O. Kristallokhimiya i stereokhimiya koordinatsionnykh soedinenii molibdena (Crystal chemistry and stereochemistry of molybdenum coordination compounds); Nauka: Moscow, 1974; pp 7-23 (in Russian). (3) Bhuvanesh, N. S. P.; Gopalakrishnan, J. J. Mater. Chem. 1997, 7, 2297-2306. (4) Inorganic Crystal Structure Database. FIZ Karlsruhe, http://www.fizkarlsruhe.de/ecid/Internet/en/DB/icsd/index.html. (5) d’Amour, H; Allman, R. Z. Kristallogr. 1976, 143, 1-13.
RostoV State UniVersity, 7 ul. Zorge, RostoV-na-Donu, 344090 Russia ReceiVed July 20, 2006 ReVised Manuscript ReceiVed December 11, 2006 CM061691M (6) Clark, C. J.; Hall, D. Acta Crystallogr. 1976, B32, 1545-1547. (7) Brown, I. D. Acta Crystallogr. 1988, B44, 545-553.
10.1021/cm061691m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007
