Trigonal capped

Using a tris(ethylenediamine) complex and ammonia as the base instead of OH-, the prospect of making a trigonal cap was conceived and achieved as follows ... 

Figure 19 Three tetrahedral Cu ions link each hexaazatriphenylene face to the bridging tetrapyridine sides of a trigonal capped cylinder. Tetrahedral metal coordination imparts a helical twist to the structure.

Figure 7.18 (a) 1,4,7,10-letraazacyclododecane triacetic acid, (LH3). (b) Structure of the 7-coordinate compiex InL] the coordination poiyhedron (shown in white) comprises a trigonal prism of 4N and 20 capped on one of its quadrilateral faces by the third O atom. 

Xc02F2 CsXeOFj KXeOjF 30.8 C21, see-saw Distorted octahedral Square pyramidal (chain) Trigonal bipyramidal (1) Capped octahedral (1) Octahedral (1)... 

MFg] , capped trigonal prismatic [MFv], and even square-antiprismatic [MFg] salts can all be isolated. By contrast with the fluorides, aqueous solutions of MCI5 and MBrs (M = Mb, Ta) yield only oxochloro- and oxobromo-complexes, though the application of non-aqueous procedures allows their use as starting materials. 

The structure of these complexes is not regular and has been described as 4 3 (C ) piano stool , which is obtained by slight distortion of a capped trigonal prism (C2d). 

Figure 25.11 Metal frameworks of some high-nuclearity binary carbonyl and carbonylate clusters of osmium (a) Os5(CO)i6 (trigonal bipyramid) (b) Os6(CO)ig (bicapped tetrahedron, or capped trigonal bipyramid) (c) [Os6(CO)ig] (octahedron) (d) Os7(CO)2i (capped octahedron) (e) [Osg(CO)22] (bicapped octahedron) (f) [Osi7(CO)36] (3 shaded atoms cap an Osu trigonal bipyramid).

Key Colour % indicates preparation but no report of colour) mp/°C (na indicates value not reported) coordination 9 ttp = tricapped trigonal prismatic 8 d = dodecahedral 8 sa = square antiprismatic 8 btp = bicapped trigonal prismatic 8,7 = mixed 8- and 7-coordination (SrBr2 structure) 7 cc = capped octahedral 7 pbp = pentagonal bipyramidal 6 o = octahedral 6 och = octahedral chain, 6 ol = octahedral layered. 

Our optimized cluster geometries agree well with other reported results for most part. One noticeable exception, however, is the case of Niia for which we find a capped distorted trigonal anti-prism structure with a central atom to be the most stable geometry, while the previously proposed distorted icosahedral structure was found to be unstable. We note that no full geometry unrestricted Optimization was done in obtaining the structure of Ref. 13. 

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