The synthesis and subsequent reduction of the arylboron dihalides 2,6-Mes(2)C(6)H(3)BX(2) (X = Cl (1); Br (2)) and 2,6-Trip(2)C(6)H(3)BBr(2) (3) (Mes = 2,4,6-Me(3)C(6)H(2)- and Trip = 2,4,6-i-Pr3C6H2-) are described. Treatment of 2 with lithium metal in diethyl ether gave the novel lithium 9-borafluorenyl compounds 4 and 5 (see Scheme 1) in which the boranediyl intermediate has inserted into an o-Me-ring C-C sigma-bond to form a borafluorenyl structure incorporating boron in a delocalized five-membered ring. Boranediyl insertion into C-C sigma-bonds, as distinct from boranediyl induced rearrangements involving C=C cleavage in delocalized aromatic substrates, is unknown. The main difference between the structures of these products is that 5 is dimerized as a consequence of the reduction in the number of solvating ethers. Reduction of 2 with KC8 led to the isolation of the 9-borafluorenyl "ate" compounds 6 and 7. These products also result from C-C bond insertion by boron as seen in 4 and 5. However, the delocalization is not observed owing to the addition of hydrogen (presumably from solvent) to the borons affording berate salts. Reduction of 3 with 3 equiv of KC8 furnishes the new "diborate" species 8. This compound features a unique B-B bonded dianionic structure with a long (1.83(2) Angstrom) B-B bond which arises from the association of two berate radical anion fragments that have a 9-borafluorenyl structure similar to those described above. Compounds 2-8 were characterized by H-1, C-13, Li-7, and B-11 NMR spectroscopy and by X-ray crystallography. Crystal data at 130 K with Cu K alpha (lambda = 1.54178 Angstrom) radiation : 1, C12H12.5B0.5Br, a = 15.562(5) Angstrom, b = 9.156(2) Angstrom, c = 16.368(5) Angstrom, beta = 111.23(2)degrees, Z = 4, space group C2/c, R = 0.060 for 1338 (I > 2 sigma(I)) data. 3, C18H14.5B0.5BT, a = 11.001(2) Angstrom, b = 12.194(2) Angstrom, c = 25.473(5) Angstrom, Z = 8, space group Pbcm, R = 0.065 for 1876 (I > 2 sigma(I)) data. 4, C32H45- BLi2O2, a = 23.611(6) Angstrom, b = 12.812(5) Angstrom, c = 10.194(3) Angstrom, Z = 4, space group Pbn2(1), R = 0.085 for 1212 (I > 2 sigma(I)) data. 5, C48H70B2Li4O2, a = 14.398(3) Angstrom, b = 17.739(4) Angstrom, c = 19.885(4) Angstrom, beta = 97.64(3)degrees, Z = 4, space group P2(1)/c, R = 0.120 for 3125 (I > 2 sigma(I)) data. 6, C32H41BKO2, a = 9.235(4) Angstrom, b = 22.292(7) Angstrom, c = 14.301(8) Angstrom, beta = 100.51(4)degrees, Z = 4, space group P2(1)/n, R = 0.074 for 2968 (I > 2 sigma(I)) data. 7 . C6H6, C33H35BK, a = 10.867(4) Angstrom, b = 12.197(5) Angstrom, c = 12.358(5) Angstrom, alpha = 108.28(3)degrees, beta = 111.76(3)degrees, gamma = 101.96(3)degrees, Z = 2, space group , R = 0.044 for 3291 (I > 2 sigma(I)) data. 8 . 1.5Et(2)O . hexane, C152H224B4K4O3.5, a = 17.686(13) Angstrom, b = 17.659(5) Angstrom, c = 23.568(10) Angstrom, beta = 104.41(4)degrees, Z = 2, space group Pc, R = 0.097 for 4672 (I > 2 sigma(I)) data.