On metal–oxygen coordination. A statistical method to determine coordination number. I. Calcium

Abstract

The environment around the Ca cation for compounds involving bonded oxygen has been studied for 254 inorganic structures containing a total of 368 polyhedra. Selection was made on the basis of the accuracy of the structural data. The coordination number (CN) was assigned using the criteria of maximum gap in the Ca--O distances and balanced bond-valence sums for Ca, but 32 cases were still difficult to assign unambiguously. A series of variables related to the Ca---O distances were calculated and averaged for each value of CN. The Gaussian curves representing the distribution of these variables for each CN overlap strongly. By way of contrast, the volume of the coordination polyhedra (Vol) showed well separated curves. Statistical analysis was applied to the set of structures with known CN, with seven variables and then with Vol alone, which seems to discriminate between the various CN groups equally well. A strong linear dependence was found for CN versus Vol. A method is proposed to assign CN in uncertain cases based on the equation: CN = 0.197 (2)Vol + 2.83 (5). Application of this equation to the unassigned cases compares favourably with discriminant analysis using the larger set of variables.

PbCa2[Al8O15] with a novel three-dimensional aluminate anion

“…The Ca 2+ cation exhibits a coordination number of 6 within a very irregular coordination polyhedron. The Ca-O distances vary between 2.2925 (13) and 2.7121 (13) Å (mean = 2.43 Å ), in good agreement with numerous other CaO 6 polyhedra and with the overall mean of the average Ca-O distances for CaO x polyhedra of 2.46 Å (Chiari, 1990). The Pb 2+ cation is situated on a mirror plane in the cavities of the B layer of the anionic framework and is bonded to seven O atoms, with Pb-O bond lengths ranging from 2.2925 (12) to 3.2809 (14) Å .…”

Mossbauer spectroscopy of Zn-poor and Zn-rich rhodonite

“…The bond-valence sums (Brown and Altermatt 1985) at M5 calculated using only the nearest six oxygen neighbors essentially satisfy the charge at M5. Further, the method of Chiari (1990) for evaluating the CN of Ca based on polyhedral volumes indicates that 6 is an unambiguously better choice than 7 for the effective coordination number of the mainly Ca-containing M5 of fowlerite. Similar calculations for the rhodonite specimens also yield 6 as the best choice of effective coordination number, although Ca is not the dominant cation in M5 of rhodonite, so the calculation may be less relevant to rhodonite than it is to fowlerite.…”

Mechanochemical Synthesis, Characterization, and Structure Determination of New Alkaline Earth Metal-Tetrafluoroterephthalate Frameworks: Ca(pBDC-F4)·4H2O, Sr(pBDC-F4)·4H2O, and Ba(pBDC-F4)

“…The last structure is very similar to the structure of compound 1 obtained from powder X-ray diffraction with small differences in cell parameters and cell volume, which could result from using two different synthesis approaches. However, the coordination number nine for Ca is rare . Though, only few compounds with a 9-fold coordination of Ca atoms are found in the literature. − A few compounds including the 9-fold coordination of strontium atoms were also reported. − …”