Seven-coordinate carbon

The Yb(III) ion in [Yb(acac)3 H2O 0.5CeH6] is seven coordinated and the coordination polyhedron for this complex is identical with the polyhedron found for Yb(l) in the unsolvated complex, i.e. a capped trigonal prism. Like the unsolvated complex, the hemibenzene solvated ones are also linked in pairs by hydrogen bonds. The benzene molecule occupies a centre of S3unmetry in the complex. All intermolecular contacts involving benzene carbon atoms and acac carbons or oxygens are above 3.5 A (84). 

The rules in overall decreasing order of importance essentially state that the ideal structures for carboranes will be based on most spherical deltahedra (rule 1) the BE hydrogens will tend to be placed in the lowest possible coordination environments (rule 2) when elements to the right of boron in the periodic table are incorporated into the deltahedron or deltahedral fragment, they will tend to preempt low-coordination sites (e.g., carbon) or, if electron-deficient, high coordination sites (rule 3) and, lastly, boron will eschew seven-coordinate BH or six-coordinate... 

As anticipated 159), the structure for CBioHn", based on rules 3p and 3s, is correct in spite of published B NMR data that 62) place the carbon in the seven-coordinate position instead of a five-coordinate position. 

There is a special and very important feature of the anticipated open nido twelve-vertex structures in Fig. 12 repetition of single Lipscomb dsd rearrangements (denoted by the two-headed arrows) monotonically allows the six skeletal atoms about the open face to rotate about the second tier of five skeletal atoms (two-tier dsd rotation). Each dsd rearrangement [85, 163) (valence bond tautomerism) recreates the same configuration and involves only the motion of two skeletal atoms (in the ball-and-stick representation) and would allow carbons, if located in different tiers, to migrate apart. Such wholesale valence bond tautomerism is known to accompany the presence of seven-coordinate BH groups, e.g., and CBjoHu 142,155). 

XIII-N12], is envisaged as most probable. In these fluxional species, two-tier dsd rearrangement would account for the production of the intermediate XII-N12 structure from XI-N12 were it necessary. Were the second carbon trapped within or restricted to the second tier (a violation of rule 3p), then the position farthest away from the seven-coordinate boron would be the best position for the second carbon (i.e.,... 

XII- N12). Actually, carbon would prefer to be about the open face, which suggests that the ideal carbon positions upon the twelve-vertex nido skeleton would be those displayed in XIII-N12 and, given the fluxional characteristics that accompany seven-coordinate BH groups, intermediate XII-N12 should be able to rearrange into the more stable... 

The structure of di-p-xylylene was first shown to be (59) by Brown and Farthing s (1949) X-ray analysis. The compound crystallizes in the tetragonal system, space group P42/mwm, with two C16H16 molecules in the unit cell. A consequence of this is that mmm molecular symmetry is a crystallographic requirement and the three carbon atoms in the asymmetric unit are completely specified by seven coordinates. The conformation of the molecule is shown in Fig. 6. If the benzene rings... 

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