Intermediate-sized boranes and their 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 nizfo-boranes  

Cluster-expansion and cluster-degradation reactions are a feature of many polyhedral borane species. Examples of cluster-expansion are  

Cluster degradation has already been mentioned in connection with the unsymmetrical cleavage reaction (p. 165) and other examples are  

Replacement of a BH unit in B5H9 by an isoelectronic organometallic group such as Fe(CO)3 or Co(tj -C5Hs) can also occur, and this illustrates the close interrelation between metalloboranes, metal-metal cluster 

The structures of several other nido- and arachno- B5-B9 boranes are given on page 154 but a detailed discussion of their chemistry is beyond the scope of this treatment. Further information is in refs. 9, 11. 27, 51 and 52. 

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