Treatment of arachno-4-Me(2)S-B(9)H(13) (1) with ethynylferrocene, FcC[triple bond]CH (Fc = ferrocenyl), in boiling benzene afforded an inseparable 2 : 1 mixture of 6-Fc-nido-5,6-C(2)B(8)H(11) (2a) and 5-Fc-nido-5,6-C(2)B(8)H(11) (2b) in a 10% yield. However, compound 2b was converted quantitatively to the pure, violet 2a by the action of proton sponge (PS), followed by acidification. The structure of 2a was determined by X-ray diffraction analysis. Heating of 2a at 400 degrees C led to the dehydrogenation and isolation of the orange 1-Fc-closo-1,10-C(2)B(8)H(9) (3) in almost quantitative yield, while treatment of 2a with t-BuNC and PS (proton sponge = 1,8-bis(dimethylamino)naphthalene) in CH(2)Cl(2) generated the neutral absolute tautomer 7-t-BuNH-8-Fc-7,8,9-C(3)B(8)H(10) (N4) (yield 50%), which could be converted quantitatively into the zwitterionic tautomer 7-t-BuNH(2)-8-Fc-7,8,9-C(3)B(8)H(9) (Z4) via dissolution in MeCN. Tautomers N4 and Z4 sharply differ in NMR properties and both gave rise to a pair of cage-isomeric closo ferratricarbollides, [1-Cp-2-Fc-12-t-BuNH-1,2,4,12-FeC(3)B(8)H(9)] (5a) (Cp = eta(5)-C(5)H(5)) (yield 48%) and [1-Cp-2-Fc-10-t-BuNH-1,2,4,10-FeC(3)B(8)H(9)] (5b) (yield 32%), upon heating with [CpFe(CO)(2)](2) in mesitylene at reflux. Electrochemical studies revealed that the compounds display one (compounds 2a and 3) or two (compounds 5a and 5b) well-defined, reversible one-electron Fe(II)/Fe(III) redox changes.