Arachno Borane anions

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

There are three main structural types, closo, nido, and arachno. Closo structures are adopted by borane anions ( = 6 -> 12),... 

Dill). A hypothetical square-planar anion 64114 (cf the isoelectronic cyclobutadiene dianion 0464 ") would have four o-bonding MO s (of symmetries Aig, Big, and Bg) and three 7r-bonding MO s (of symmetries Am and Eu), which would be stabilized and modified in symmetry, but not added to, if 2 6H + units were brought up the fourfold axis to complete the octahedron of 68Hb . Significantly, the neutral arachno-borane 64640 actually has a C v arrangement of its 4 skeletal boron... 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arachno-horanes tend to be more reactive (and less stable to thermal decomposition) than nWo-boranes and reactivity also diminishes with increasing mol wt. C/oso-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

Boranes and carboranes have structures in which their skeletal B- or C-atoms form triangular-faced polyhedra. There are basically three structural types, namely the closo- (an euphonious modification of the Greek clovo = cage, i. e., a complete or closed polyhedron), the nido (from Latin nest-like ) and the arachno- (from Greek cob-web ) structure. Each of these three types is adopted by cluster compounds of specific atomic ratios. c/o o-Structures occur in borane dianions B H , in car-borane anions (CB iH ) , and carboranes (C2B 2H ). Each skeletal atom has a single H-atom terminally attached by a bond directed outwards, away from the polyhedron center (see the example of BioHio in Fig. 3-1 below). Wo-Structures are adopted by boranes B H +4 and their related carboranes CB iH +3, C2H 2H +2 etc., and amc/z/2o-structures by boranes B H +6 and related carboranes CB iH +5, C2B 2H +4 etc. In other words, carboranes have the general formula [(CH) (BH) Hc] , where the sum a + c + x) is equal to 2 for a closo-structure, 4 for a /do-structure, and 6 for an amc/z o-structure. 

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. ... 

For a B Hm borane based on an anion B H 6-, an arachno-structure is preferred with B at n vertices of an (n + 2) vertices polyhedron (n + 3) pairs of framework bonding electrons are required. 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

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