Boranes nido-structures

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

Application of the Wade-Mingos Rules to Polyhedral Boranes Nido and Arachno Borane Structures... 

The structures of boranes can be grouped into several classifications. If the structure contains a complete polyhedron of boron atoms, it is referred to as a closo borane (closo comes from a Greek word meaning closed ). If the structure has one boron atom missing from a comer of the polyhedron, the structure is referred to as a nido borane (nido comes from a Latin word for nest ). In this type of structure, a polyhedron having n comers has (n - 1) comers that are occupied by boron atoms. A borane in which two comers are unoccupied is referred to as an arachno structure (arachno comes from a Greek word for web ). Other types of boranes have structures that are classified in different ways, but they are less numerous and will not be described. 

In addition, there is a wide range of borane derivatives containing atoms of other elements, such as carbon and phosphorus. Borohydrlde ions of the type BeHe also exist. Boranes and borohydrlde ions are classified according to their structure. Those with a complete polyhedron are said to have a closo-structure. Those in which the polyhedron is incomplete by loss of one vertex have a nido-structure (from the Greek for nest ). Those with open structures by removal of two ot more vertices have an arachno- structure (from the Greek for spider ). See also Wade s rules. 

Determine the type of borane cluster from Table 10.20, where n is the number of B atoms. Goso structures have n vertices. Nido structures (from the Greek word for nest) are based on the n + I deltahedron with a single vertex missing. Similarly, the arachno (spider), hypho (net), and klado (branch) boranes... 

The boranes nido-B and m do-BioHi4 and carbaboranes n/do-2,3-Et2C2B4H6 and c/oro-l,2-C2BioHi2 have been used as precursors for boron and boron carbide thin films. ISEELS has been used to characterise the molecular precursws at solid surfaces a comparison has been made between the bond lengths determined for the molecular clusters by MNDO calculations and results from X-ray or electron diffraction and from extended EELS fine structure. A study of the deposition of boron carbide films on Si(lll) surfaces from the synchrotron radiation induced decomposition of nido-2,3-diethyldicarbahexaborane has been described. The dissociation of the cluster is the rate determining step in the process. As with the deposition of boron from nido-BioHi4, it is found that thoe is an activation barrier to the dissociation of the boron carbide precursor on Si(l 11) surfaces.43... 

We have seen what happens in a borane cluster if there are not enough BH fragments to fill the vertices we get a nido structure with an empty vertex. The same is true for transition metal clusters, for example, in Fe5(CO)i5C, the carbon atom, which is not considered as a vertex atom, is encapsulated within the cluster and gives all its four valence electrons to the cluster. The Fe(CO)3 fragment contributes two cluster electrons as it is isoelectronic with Os(CO)3. The total count is therefore (5 x 2) + 4 = 14, and the number of vertices is -1 = 6. This requires the structure shown as 13.18, as is observed for this and the analogous Ru and Os species. 

A second type of borane clusters are those with the general formula B H +4. These are classified as nido bo-ranes. Isoelectronic carboranes of the general formula C cB H +4 are produced by replacing a BH unit with a C. Nido refers to the nest-like structure of these compounds. Such a structure is obtained by the removal of one vertex from a closo structure (Fig. 22). Compounds of the general formula, B H +4 will have n electron pairs (one from each BH unit) plus four electrons or two pairs from the four additional hydrogen atoms. The total of n- -2 electron pairs will be used for skeletal bonding. Generally, any cluster compound with n -I- 2 skeletal electron pairs will adopt a nido structure. 

Barton et al. reported an interesting series of Os-Ir boranes complexes with a pileo-closo-nido structural sequence. [Os(BsH9)(PPh3)2GO] was reacted with BuLi and a 0.5 equiv. of [(Gp IrCl2)2] in THF at —35 °G to give complexes 90-92. ... 

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