Hypho boranes

No. of Skeletal Bonding Pairs Basic Polyhedron closo- Borane nido- Borane arachno- Borane hypho- Borane... 

B H +g. No neutral borane has yet been definitely established in this series but the known compounds BgHie and BioHig may prove to be hypho-hormcs and several adducts are known to have hypho-stmctures (pp. 171-2). 

Boranes are usually named by indicating the number of B atoms with a latin prefix and the number of H atoms by an arabic number in parentheses, e.g. B5H9, pentaborane(9) B5H11, pentaboranefl 1). Names for anions end in ate rather than ane and specify both the number of H and B atoms and the charge, e.g. BsHs" octahydropentaboratefl—). Further information can be provided by the optional inclusion of the italicized descriptors closo-, nido-, arachno-, hypho- and conjuncto-, e.g. ... 

A large number of borane clusters of the hypho-type are known that contain bridging groups such as CH2, NH, S, etc. Formally, these groups could be considered to... 

For a B Hm borane based on an anion B H 8-, a hypho-structure is preferred. B is at n vertices of an (n + 3) vertices polyhedron requiring (n + 4) pairs of framework bonding electrons. 

The hypho (Gr.. net ) series of boranes, with electronic formula 2n + 8, has beer suggested to augment the closo, nido, and arachno series. Although no neutral boranes fit this scheme, some borane derivatives do. It is also passible to construct units consisting of more than one of the above types. These are called cmjuncio (Latin, joined subunits ) structures. 

For boranes and carboranes, the terms do so, nido, arachno, and hypho are used to describe their molecular skeletons with reference to a series of increasingly open deltahedra. Boranes of the nido, arachno, and hypho types are compounds based on formulas B H +4, B H +6, and B Il +8, respectively. The structures... 

The polyhedra which represent the c/ow-boranes also serve as the basis for the structures of the nido-, arachno- and hypho-boranes. The term nido, as mentioned... 

A fourth class of boranes, the hypho-boranes, relates to the above species in that the polyhedron, composed of n vertices, contains n + 4 skeletal electron pairs... 

Table 1. Examples of closo-, nido-, arachno-, and hypho-Boranes...

Table 1 shows examples of closo-, nido-, arachno- and hypho-boianes containing 6 through 13 skeletal bonding electron pairs. Where neutral borane example are not available, ionic species or heteroboranes are cited. 

Finally, several authors including Lipscomb himself, have pointed out the relationship of the styx rules (see above) to Wade s rules55,87,88. The sum of the digits in the topological styx formula for a borane structure gives the number of skeletal electron pairs in the molecule since both approaches factor out the bonds to exopolyhedral substituents. Thus, in the general formula BpHp+q used in the styx rules, q values of 2, 4, 6 and 8 are identified with closo-, nido-, arachno-and hypho-borane structures, respectively. 

This section presents the structure of the boron hybrides and is arranged in accordance with the relationship defined by Wade s Rules and expressed by Williams and Rudolph. Thus for the boranes containing six or more pairs of skeletal bonding electrons, the relationship between the structures of the closo-, nido-, arachno-and hypho-species is described. In cases where the parent borane does not exist, examples from heteroboranes with the correctly predicted structure based on Williams coordination number pattern recognition theory (CNPR) of borane structures will be described 70. Treated separately will be mono- and diborane species and also species with more than 12 boron atoms. Although there have been several reviews on the structures of the boranes in recent years none have used the current approach89. ... 

A hypho-borane adduct, B3H53P(CH3)3 has been prepared recently which belongs to the N = 7 class 1U). Boron-11 NMR data support the structure shown in Fig. 14. 

Special prefixes closo-, nido- arachno-, catena-, hypho-, dado-, isodoso-, isonido-, isoarachno-, canasto-, anello-, predoso-, hyperdoso-, pileo-, and conjuncto- are found in the literature to describe borane structures. 

Together, the closo stractures make up only a very small fraction of all known borane species. Additional structural types can be obtained by removing one or more comers from the closo framework. Removal of one comer yields a nido (nest-like) stmc-ture, removal of two comers an arachno (spiderweb-like) stracture, removal of three comers a hypho (net-Uke) stracture, and removal of four comers a klado (branched) structure. Examples of three related closo, nido, and arachno borane structures are... 

As we have seen previously, the number of electrons involved in framework bonding in boranes is related to the classification of the structure as close, nido, arachno, hypho, or klado. Rearranging this equation gives... 

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