ChemInform Abstract: Metal‐Metal Interactions, a Pairing Distortion, and S‐Lined Channels in the M2Ta9S6 Structure

Abstract

The metal-rich sulfides M2Ta9S6 (M = Co, Fe, Ni)1-2 present us with a very interesting three-dimensional intermetallic structure containing one-dimensional channels sheathed by sulfur atoms. A comparable structure is also observed for the selenides M2TallSe8 (M = Co, Fe, Ni),3 where the selenium channels differ in size and geometry. The bulk of the M2TagS6 structure consists of tantalum trigonal prisms, with M in the center and capping tantalum atoms in the rectangular faces. These units are stacked, are parallel to the channels, and are linked together by the capping tantalum atoms and sulfur atoms. This arrangement is interesting in that it differs markedly from that found in other tantalum sulfides. As a matter of fact, not only for the binary sulfides Ta6S4 and Ta2SS but also for the early-transition-metal derivatives M,Ta6-xS (M = V, Cr; x i= 1)6 analogous columns consist of face-sharing pentagonal antiprisms or interpenetrating distorted icosahedra. These are connected by sulfur atoms, but channels are only observed in the structure of Ta2S. The situation changes drastically when moving toward less metal-rich sulfides, as different structures are found. For instance, Ta2NiS2 and TalNiSe77b,8 are layered compounds, and the coordination around both Ta and Ni has changed. Another striking characteristic of the ternary sulfide M2Ta9S6 structure is the different behavior exhibited by nickel on the one hand and cobalt and iron on the other. Ni2TagS6 is more symmetric with metal chains having equally spaced nickel atoms (3.37 A apart). In cobalt and iron derivatives some pairing occurs and two different alternating M-M distances are found (they are respectively 2.88 and 3.70 A for iron and 2.90 and 3.67 A for Co), along with some distortion in the tantalum trigonal prisms. In the present study, we shall try to understand the bonding in these solids and the reasons that lead to the observed distortions in some of them. The possible introduction of other atoms in the channels and their interaction with the sulfur atoms will also be examined. The calculations used are of the extended Hiickel type? using the tight-binding method.’O Details are given in the Appendix. Structure of Ni2Ta& This compound crystallizes in the hexagonal space group P62m. A projection on the hexagonal plane of the atoms in a few unit cells is shown in Figure 1 (there is one formula unit per unit cell), revealing clearly the existence of empty spaces, which form the