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In earlier publications from our laboratory, methods were described for the regiospecific introduction of organic substituents onto cage boron and carbon atoms in small metallacarborane clusters. Here we report the selective application of these approaches in the targeted synthesis of several double-decker, triple-decker, and tetradecker sandwich compounds including the parent species ( C S M~~) C O ( C~B~H~) and ( C S M~~) C~( C~B~H S ) C O ( C~M~S ) . The peralkyl species (C5Me5)Co11'[(Me2C2B3Me3(p-H)2] (51, an isostructural and isoelectronic analogue of the decamethylcobaltocenium ion [ (C5Me5)2C0111]+, was prepared and structurally characterized via X-ray diffraction. The CzB3 and C5 rings in 5 are staggered and nearly parallel, and the metal is approximately centered over each ring. The cobalt-CsMes centroid distance (1.68 A) in 5 is virtually identical to that reported in the literature for (C~Me5)2Co+, but the cobalt-CzB3 centroid vector is much shorter (1.54 A), indicating greater covalent character in the (2-rborane vs the C045Me5 interaction. In 5 the C5Me5 methyl groups are bent out of the C5 plane away from the metal, in accord with theoretical predictions, but the methyls on the carborane ligand are bent toward the metal atom, an observation that probably reflects differences in orbital hybridization in the two rings. The permethyl triple-decker complex (C5Me5)Co(Me2C2B3Me3)Co(C5Me5) was synthesized via deprotonation and metal-capping of ( C !~M~~) C O ( M~~C~B J M~~H~) . The unsubstituted triple-decker complex (C~M~~)C~(C~B~HS)CO(C~M~~) was prepared via two routes: (i) desilylation of the C,C'-bis(trimethylsily1) derivative and (ii) deprotonation and metal capping of the parent double-decker ( C~M~~) C O ( C~B~H~) . Attempted tetradecker stacking of (C5Mes)Co-(C2B3H5-5-Cl)-ion with NiBr2 led to the fusion product (C5Me5)2C&4B6H&l~, but ( C~M~~) C O ( M~~C~B S H~-~and NiBr2 gave the desired tetradecker [(C~M~~)CO(M~~C~B~M~HZ)]~N~ in low yield. New compounds were isolated as air-stable solids and characterized via 'H and IlB NMR, IR, and mass spectra. Supplemeatuy Material Avdhbk: Tables of anisotropic thermal parameters and mean planes for 5 (2 pages). Ordering information is given on any current masthead page. (1 5 ) TEXSAN: single Crystal Structure Analysis software, Version 5.0; Molecular Structure Corp.: The Woodlands, TX 77381, 1989.

The idea of using students or groups of students to lead a discussion is nothing new in chemistry classes or other academic fields (1-5). A sustained effort to actively engage students in lecture classes has been going on in the sciences for at least a decade (6-11), with pros and cons about such teaching being voiced by both faculty and students (12)(13)(14). I wish to report on a student group teaching technique that has been used successfully in the general chemistry lecture at the 100-level. This technique has received high marks in student course reviews and should enhance student learning at any level.

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