Five-vertex structures

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). 

The structures of 13 (33), 14 (34), and 15 (35) are shown in Figs. 13, 14, and 15. Each of the products is a 76-electron five-vertex cluster in 13 the phosphorus ligand donates three electrons to the cluster, 14 is the trimethyl-phosphite substitution product of 13, and in 15 the phosphorus ligand contributes five electrons to the cluster. As with the isoelectronic [Os5C(CO)l5I]-, the Os5C cores take open structures based on a pentagonal bipyramid in which only three of the equatorial vertices are present. The structures of the Os5C cores are very similar for 13,14, and 15. Mean Os-Os distances are 2.90 0.06 A for all three, and the carbido carbon atom is... 

Os5(CO),6 N is found to be 9 for this complex, and the only five-vertex figure with nine edges is a trigonal bipyramid the observed structure has a slightly-distorted trigonal bipyramidal arrangement of Os atoms. 

FIGURE 87 Structure of the pentadecanuclear cluster [EU)5(/t3-OH)2o(/t5-Cl) (/t3-Tyr)io(OH)2(/i-H20)5(H20)i8] (left). The wheel-like core structure showing five vertex-sharing cuboid [Eu4(/t3-OH)4] units centered on a /ts-Cl ion (right) (redrawn after Wang et al., 2001b). 

What happens when there are more atoms than vertices into which they can fit For example, Os6(CO)ig is a (6 x 2) = 12 cluster electron species. This means that the number of vertices required by Wade s rule is - l 5. The structure found for the molecule, 13.16, shows that the extra metal atom bridges to a face of the five-vertex base polyhedron, and so is able to contribute its electrons to the cluster, even though it cannot occupy a vertex. 

In these cases, I-, II-, and III-Al 9, conservation of the 3-center bonds would require five (or six) 3-center bonds to be expressed within the skeleton matched with four (or three) 3-center bridge hydrogens, respectively. There are several examples demonstrating that the nine-vertex arachno system is flexible enough to adapt to both electronic alternatives. The structures for i-BgHjgrLB (X-A19) and i-B9Hi4 (A-VI9) are both known in the encumbered crystalline phases and both must accommodate six 3-center bonds within the skeletal framework. By contrast, i-C2B7Hx3 (II-A19) absorbs only five 3-center bonds within the skeletal framework. 

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