Prismatic cavities

Fig. 35. Co,3C2(CO)fc, 28, as in its (PhCH2N(CH3)3)+ salt (68). The two encapsulated carbon atoms occupy trigonal prismatic cavities, which share a common vertex [Co(l)]. The two prisms, which are outlined with solid lines for clarity, are rotated with respect to one another (see text), and two further cobalt atoms [Co(4) and Co(5)] assume capping positions. Average Co-Co distance = 2.57 A average Co-C distance = 1.98 A.

Figure 3.75. Three-dimensional microstractures (a) cubic trenches (top Mly developed bottom not fully developed), (b) prismatic trenches (top fully developed bottom not fully developed), and (c) prismatic cavities of 100 pm width, 20 pm length, and 20 pm depth, which are connected by twelve channels of 50 pm length and 4 pm x 4 pm cross section bearing a periodicity of 8 pm and lying 10 pm below the surface. (From Ref. [264] with permission of Wiley-VCH.)...

Fully developed 3D microstructures are depicted in Figure 3.77. However, edges of the positive-resist structures are rounded in Figure 3.75. This is mainly caused by proton diffusion during the postexposure step [264], The prismatic cavities shown in Figure 3.75c may be a possible approach for switchable grating devices if they are filled with a low molecular weight liquid crystal. 

As would be expected, there are exceptions to these rules, for example phosphorus has been located in a trigonal prismatic cavity in the cluster [Os6P(CO)ig] f l and arsenic in a square antiprismatic cavity in [RhioAs(CO)22] , even though theoretically it seems unlikely. The aim of this section is to hi ight the different interstitial sites in which main-group atoms have been located, and to illustrate the effects they have on the polyhedral cluster core. 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

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