The syntheses of new cyano-substituted derivatives of arachno-6,8-C2B7H13 have been achieved through the addition reactions of the arachno-6,8-C2B7H12- (1(-)) anion with cyano-activated olefins. The reaction of PSH(+)1(-) with tetracyanoethylene (TCNE) yielded the unusual bridging compound PSH(+)endo-6-endo-7-[mu2-(C(CN)(2))(2)]-arachno-6,8-C2B7H12_ (PSH(+)2(-)) resulting from cycloaddition of the TCNE at the C6-B7 edge of the anion. Consistent with its hypho skeletal electron count, an X-ray crystallographic study and DFT/GlAO calculations confirm 2(-) has a more open structure than 1(-). The reaction of 1(-) with acrylonitrile resulted in the formation of endo-6-(NCCH2-CH2)-arachno-6,8-C2B7H11- (3(-)), which, upon acidification, afforded endo-6-(NCCH2CH2)-arachno-6,8-C2B7H12 (3) in high yield. X-ray crystallographic and DFT/GlAO studies established that the cyanoethyl fragment in 3 is substituted at the endo-position of the C6 cage-carbon. Heating 3 in THF at 50 degreesC or in toluene at 110 degreesC resulted in the quantitative isomerization of the cyanoethyl-substituent from the endo- to the exo-position at C6 to yield exo-6-(NCCH2CH2)-arachno-6,8-C2B7H12 (4). This is the first example of an endo to exo isomerization to be observed at a cage-carbon of a carborane. While heating 3 resulted in isomerization to 4, heating 3(-) in the presence of a small amount of 3 yielded the new ethylene-bridged 10-vertex tricarbaborane mu(6),(9)-(CH2CH2)-arachno-5,6,9-C3B7H11 (5) resulting from reduction of the 3(-) pendant nitrile group, followed by deammination and carbon insertion.