Vertex sites

In addition to the C4V structure of the M(CO)3E9 ions shown in Fig. 5, a second isomer has been characterized for both the tin and lead clusters. Sn9W(CO)3" and Pb9Mo(CO)3 have both been isolated in a Cs structure in which the M(CO)3 fragment occupies a 5-coordinate waist vertex site instead of the 4-coordinate capping site shown in Fig. 5 [55, 60]. Both of these isomers give the same NMR spectra, which is consistent with the higher symmetry 4 structures. We interpret these data to indicate that there is local exchange between the 4-coordinate and 5-coordinate sites, but this interpretation is controversial [34, 55]. 

Note The carbido carbon atom is semi-interstitial and is regarded as a ligand and not a vertex site, as shown below ... 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

If the square pyramidal metal carbonyl carbides Fe5(CO)i5C ° and Os5(CO)i5C are treated in a similar manner to I xyi ( ()) i T that is, as clusters in which all four of the core carbon atom s valence shell electrons are used for skeletal bonding, then they are seen to have the expected nido shapes of systems with five skeletal atoms (the metal atoms) held together by seven skeletal bond pairs. By contrast, if these carbide carbon atoms had occupied polyhedral vertex sites, with a lone pair of electrons in an exo-oriented sp hybrid orbital, then the number of skeletal bond pairs would have been reduced by one and the number of skeletal atoms would have increased by one. The five metal atoms and the carbide carbon atom would have had to be accommodated in some way on a trigonal bipyramidal skeleton. Clearly, the assumption that all four valence shell electrons from the carbide carbon atom are involved in the skeletal bonding is vindicated. 

Vertex site (1) target, biased flow (see text). 

Finally, if one relaxes the constraint that the coreactant is injected into the system at a specific site and instead assumes that the coreactant can initiate its motion at any site i of the reaction space, some interesting new effects arise. For the case of unconstrained motion, the results calculated for the overall n) show that placing the reaction center at the midpoint base site (with i> = 4) results in a much more efficient diffusion-reaction process than placing the trap at a defect (vertex) site (with v — 2). On the other hand, biasing the motion of the diffusing coreactant (e.g., by switching on a... 

---