Poeppelmeier, Corbett
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(36) C. H. Wei and L. F. Dahl, J. Am. Chem. Soc., BO, 3977 (1968). (37) P. Chini, Inorg. Chim. Acta Rev.. 2, 31 (1966). (38) A bonding model applicable to briding carbonyls is discussed by
and Curtiszo
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Synthesis and Structure of ScsCl8
Triplett
(39) M. H. Chisholm, F. A. Cotton, B. A. Frenz, W. W. Reichert. L. W. Shive, and B. R. Stults, J. Am. Chem. SOC.,98, 4469 (1976). (40) M. H. Chisholm, F. A. Cotton, and M. Extine, J. Am. Chem. SOC.,g8,4477 (1976).
Cluster Condensation Reactions. Synthesis and Structure of Pentascandium Octachloride (SqClS). An Infinite Chain Structure Derived by Cluster Condensation Kenneth R.Poeppelmeier and John D. Corbett* Contribution from the Ames Laboratory-USDOE and Department of Chemistry, Iowa State Uniuersity, Ames, Iowa 5001 I . Received December 30, 1977
Abstract: Single crystals of the phase pentascandium octachloride, Sc&ls, are obtained in several weeks by chemical transport from 940 to 960 OC in a sealed tantalum container starting with powdered metal and ScC13. The role the lower valent gasphase species ScC12(g) plays in the formation of scscl8 at high temperature is discussed in terms of the postulated transport reaction Sc5Cl~(s) 2ScCl3(g) = 7ScClz(g). The structure of the green-hued, black crystals has been determined with fullmatrix least-squares refinement of all atoms with anisotropic thermal parameters. This phase crystallizes in the monoclinic space group C 2 / m with a = 17.78 (2), b = 3.523 (8), and c = 12.04 (1) A, /3 = 130.10 (6)O, and 2 = 2; final residuals were R = 0.1 15 and R, = 0.136 for 486 independent reflections with 28 I 50' and I > 3 0 ( 1 ) . Structurally Sc5Cl8 contains separate infinite chains of metal octahedra which share trans edges as in GdzC13; the common edges contain the shortest Sc-Sc bond yet observed, 3.021 (7) A. A second, parallel chain consists of edge-shared octahedra of chloride about isolated scandium(II1) atoms. The connectivity description [ S C ( C ~ ) ~ ~ ~ ( C ~ ) ~ ~ ~ ] , [ S C ~ Cis I well ~ ( Capproximated I ) ~ ~ ~ ( C ~by ) ~ (ScC12+),~~], ( S C ~ C I ~ -the ) ~ ,latter emphasizing the anionic nature of the metal chain. The structure can be considered as arising from the formal condensation of MsXl2 clusters such as are found in Sc7C112.The anionic character of the metal chain allows the metal atoms therein to be formally more reduced than in Gd2C13, an effect which is reflected in the relative shortening of the bonds in Sc5Cls. The sums of metal-metal bond orders in four structurally diverse scandium chlorides as well as in GdzCI, are found to be a smooth function of the number of delocalized electrons therein. Important in the variety of scandium chloride structures found with small changes in composition is the varied role halogen atoms serve in connectivity together with the ability of scandium to generate isolated cations and thence strongly metal-metal bonded anionic arrays.
+
Introduction Evidence that any rare-earth metal, yttrium, or scandium would reduce its corresponding trihalide to compositions with X:M < 2.0 has until very recently been limited to the single example of the unusual gadolinium compound G d ~ C l 3 ~where 3~ a remarkable infinite metal chain consisting of edge-shared octahedra is formed. Later McCollum and co-workers3 were able to reduce scandium trichloride and tribromide to the composition Sc2X3 with scandium foil at elevated temperatures when proper attention was paid to avoiding blockage of the metal surface by product, a condition which also allows the preparation of the monochloride ScCl which contains double metal layers4 Recent applications of the technique of chemical transport along a temperature gradient above 880 OC have now allowed kinetic accessibility to several additional scandium phases in the composition range 2.0 > CI:Sc > 1 .O. Among these are Sc7Cll0,~with infinite metal chains composed of double octahedra, S C ~ C Iwhich ~ ~ , contains ~ discrete six-atom metal clusters, and the compound of interest in this work, Sc5Clg. The chemistry of very reduced rare-earth metal halides (X:M < 2 ) has also recently been broadened substantially by Simon and coworkers,' who have reported the formation of monochlorides for M = Gd, Tb, Er, and Lu together with Tb2C13 which is isostructural with Gd2C13. This paper reports on the synthesis, single-crystal x-ray structure, and electron paramagnetic resonance behavior of Sc5Cls and relates these results both in terms of architecture and electronic states to those of other polyscandium structures. The formation of 0002-7863/78/1500-5039$01 .OO/O
Sc$& and other transported compositions is considered in terms of transport by the high-temperature species ScCl2(g).
Experimental Section Synthesis. All preparation and analyses of materials were as described in an earlier p ~ b l i c a t i o nThe . ~ best metal available (99.998 atomic % purity) was used and as such had impurity levels in atomic parts per million as follows: H, 401; C, 105; 0, 118; F, 14; Fe, 50; Ni, 20; Cu, 1.5. The formation of compounds which are very incongruent melting from either a melt or the metal by a reaction requiring substantial diffusion certainly is kinetically hindered, although thermodynamically the compounds appear quite stable. The disproportionation of a lower valent vapor species such as ScC12 would, on the other hand, provide a direct and facile transport reaction, viz., Empirical evidence based on observation suggests that there are several competing processes and that, depending on conditions, compounds as oxidized as Sc7C112and as reduced as the metal itself will deposit. Simple condensation of a stable dichloride has not been observed. A possible structure for ScClz(s) would be the neutral cluster sC6c16ic112/2i-u, isostructural with the recently characterized Z T ~ I ,and ~ , ~an energetically more favorable alternative to the layered CdI2 or CdCl2 types common to the majority of the transition metal dihalides. However, only the nine-electron cluster [Sc3+][SC&16'C1,2j2'-u]3- has been prepared to date, and the stoichiometric dichloride with only six bonding electrons may be too electron deficient to be stable.
Acknowledgments. This work was supported by the U S . Department of Energy, Division of Basic Energy Sciences. The authors wish to thank F. Laabs for his excellent analytical microprobe results and Professor R. A. Jacobson for his assistance in the use of the diffractometer. Supplementary Material Available: Tables of structure factor amplitudes (3 pages). Ordering information is given on any current masthead page.
References and Notes (1) J. E. Mee and J. D. Corbett, Inorg. Chem., 4, 88 (1965). (2) D. A. Lokken and J. D. Corbett, Inorg. Chem., 12, 556 (1973). (3) 9. C. McCollum, M. J. Camp, and J. D. Corbett, Inorg. Chem., 12, 778 (1973). (4) K. R. Poeppelmeier and J. D. Corbett, Inorg. Chem., 16, 294 (1977). (5) K. R. Poeppelmeier and J. D. Corbett, Inorg. Chem., 16, 1107 (1977). (6) J. D. Corbett, R. L. Daake, K. R . Poeppelmeier,and D. H. Guthrie, J. Am. Chem. SOC.,100, 652 (1978). (7) A. Simon, Hi. Mattausch,and N. Holzer, Angew. Chem., /nt.Ed. Engl., 15, 624 (1976). (8) W. J. Rohrbaugh and R. A. Jacobson, Inorg. Chem., 13,2535 (1974). (9) R. A. Jacobson, J. Appl. Crysra//ogr., 9, 115 (1976). (10) D. G. Adolphson and J. D. Corbett, Inorg. Chem., 15, 1820 (1976). ( 1 1) D. G. Adolphson, J. D. Corbett, and D. J. Merryman, J. Am. Chem. SOC., 96, 7234 (1976). (12) The letters following atom numbers index the generating symmetry operations, Table 11. (13) W. Klemm and E. Krose, 2.Anorg. Allg. Chem., 253, 218 (1947). (14) H. Schafer and H. G. Schnering, Angew. Chem., 76, 833 (1964). (15) L. Pauling, "The Nature of the Chemloai Bond", 3rd ed,Cornell University Press, Ithaca, N.Y., 1960, pp 400, 410. (16) Following the nomenclature introduced in ref 14, i = inner and a (ausser) = ex0 (outer)halogen atoms which are nonlinking and therefore belong only to one metal cluster. Those forming links between two clusters are X'-/ or X s - a depending on the mode. (17) A. Simon: Chem. UnsererZeit, IO, l(1976). (18) J. W. Hastie. R. H. Hauge, and J. L. Margrave, High Temp. Sci., 3, 257 (1971).
