Bond distances ternary compounds

Recently, Hoffmann et al. (1989) reported a series of the ErgRhjCij-type compounds present in the R-Rh-C system. These compounds RgRhjCjj (R=Y, Gd-Tm) are thermodynamically stable at 900°C and have a monoclinic structure, space group C2/m, with Z — 2 formula units per unit cell. The lattice parameters of these compounds have been measured by single-crystal X-ray diffraction. The structure contains a finite chain-like centrosymmetric polyanion [Rh5Ci2] with two Rh-Rh bonds (2.708 A) and six pairs of carbon atoms. The shortest distances between adjacent Rh5Ci2 clusters are the Rh-C distances of 2.94 A and the Rh-Rh distances of 3.27 A, and thus the Rh5Ci2 units may be treated as isolated from each other. This structure is characterized by three different kinds of C2 pairs. The C-C bond distances of 1.27, 1.32 and 1.33 A are between those of a triple bond (1.20 A) and a double bond (1.34 A) in hydrocarbons, and are the shortest found so far in ternary carbides of the rare earth metals with transition metals. 

Table S5.15 shows the interatomic distances in ternary compounds. In all cases, compounds with identical are compared to exclude its influence on the bond lengths. It is obvious that on variation of M in compounds MnAO(F)m the increase of x(M) or reduction of d(M—X) decrease the electron density in the A-O(F) bond that increases its length.

On the base of the analysis of the interatomic distances and the coordinations of atoms it was concluded that the Snl, R and Rh atoms have a behaviour which is similar to cations, the Sn2 atoms have a behaviour which is similar to anions, and in the compound there is an ionic-covalent type of the bond (Hodeau et al. 1980, Miraglia et al. 1986). The authors draw a parallel between the SniR3Rh4Sni2 structure and the stmcture of the ternary oxides A A"B40i2 similar to perovskite. 

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