Contrasts in Structural and Bonding Representations among Polar Intermetallic Compounds. Strongly Differentiated Hamilton Populations for Three Related Condensed Cluster Halides of the Rare-Earth Elements

The crystal and electronic structures of three related R(3)TnX(3) phases (R = rare-earth metal, Tn = transition metal, X = Cl, I) containing extended mixed-metal chains are compared and contrasted: (1) Pr3RuI3 (P2(1)/m), (2) Gd3MnI3 (P21/m), and (3) Pr3RuCI3 (Pnma). The structures all feature double chains built of pairs of condensed R-6(Tn) octahedral chains encased by halogen atoms. Pr3RuI3 (1) lacks significant Ru-Ru bonding, evidently because of packing restrictions imposed by the large closed-shell size of iodine. However, the vertex Pr2 atoms on the chain exhibit a marked electronic differentiation. These are strongly bound to the central Ru (and to four l), but very little to four neighboring Pr in the cluster according to bond populations, in contrast to Pr2-Pr "bond" distances that are very comparable to those elsewhere. In Gd3MnI3 (2), the smaller metal atoms allow substantial distortions and Mn-Mn bonding. Pr3RuCI3 (3), in contrast to the iodide (1), can be described in terms of a more tightly bound superstructure of (2) in which both substantial Ru-Ru bonding and an increased number of Pr-CI contacts in very similar mixed-metal chains are favored by the smaller closed-shell contacts of chlorine. Local Spin Density Approximation (LSDA) Linearized Muffin-Tin Orbital (LMTO)-ASA calculations and Crystal Orbital Hamilton Population (COHP) analyses show that the customary structural descriptions in terms of condensed, Tn-stuffed, R-R bonded polyhedral frameworks are poor representations of the bonding in all. Hamilton bond populations (-ICOHP) for the polar mixed-metal R-Tn and the somewhat smaller R-X interactions account for 75-90% of the total populations in each of these phases, together with smaller contributions and variations for R-R and Tn-Tn interactions. The strength of such R-Tn contributions in polar intermetallics was first established or anticipated by Brewer. The rare-earth-metal atoms are significantly oxidized in these compounds.