The red-brown tricobalt compound 2d (43e) can be conveniently prepared by reaction of protonated 3qs-cyclopentadienyl(q5-1,3-diborolene)cobalt151 -with (C,H,)Co(CO), in mesitylene at 170°C [74% yield, m.p. 209°C; S'H(C6D6): 14.6, 9.3, -10.0, -12.9, -38.1; m/z=601 (M+, 100°/o rel. int.)]. Silver ions oxidize 2d to the dark-red, diamagnetic 42e cation 2d+ [78% yield; G'H(CDzC12): 1.32 (s, 6), 1.27 (t, 6), 2.19 (s, I), 2.8 and 2.1 (m, 4), 4.65 (s, 5 ) ; 6"B: 18.31.These results reveal the unusual ligand properties of 1,3diborolene for the construction of tetradecker complexes containing 42 to 46 valence electrons. With exception of the paramagnetic 2c, with "high-spin" d6 Fe, the new trinuclear complexes are electronically closely related to the corresponding mono-and dinuclear complexes : Formal insertion of the 12e-fragment 1,3-diborolenylcobalt [(CzB2C)Co] between CsHS and Ni in nickelocene affords 1, and then 2a. Since the 17e-sandwich .radical [(C,H,)CO(C~B,C)]~ corresponds to the 5e-ligand C,HF, a nickelocene-like chemistry is to be expected, at least in the case of the triple-decker 1.