Na[B10H13] reacts with 9-Br-BBN (9-BBN = 9-bora[3.3. 1]bicyclononane) in CH2Cl2 to produce (9-BBN)B10H13, 1, in high yield. Single-crystal X-ray diffraction analysis shows that the baron atom of the 9-BBN unit in 1 is partially inserted into the nido-B-10 structure through a highly asymmetric bridge between B(5) and B(6) on the B-10 cluster with distances of B(BBN)-B(5) = 1.740(6) Angstrom and B(BBN)-B(6) = 2.079(6) Angstrom. There is also an agostic interaction of the terminal H(5b) on B(5) with B(BBN). NMR studies confirm that this interaction is maintained in solution. Compound 1 is unstable in coordinating solvents, resulting in some cases in the cleavage of the 9-BBN unit from the decaborane skeleton. When 1 is placed in Et2O, NMR spectra suggest that (Et2O)-(9-BBN)B10H13, 2, is formed in which an Et2O molecule associates with the baron B(BBN) of the 9-BBN unit while the interactions of that boron with B(6) and with H(5b) are eliminated, leaving only a direct bond between B(BBN) and B(5). Reaction of 1 with "Proton Sponge", 1,8-bis(dimethylami no)naphthalene, in CH2Cl2 gives [PSH][(9-BBN)B10H12], 3, (PSH = protonated "Proton Sponge"). Single-crystal X-ray diffraction analysis of 3 indicates a structure similar to that of nido-[B11H14](-), With the baron of the 9-BBN unit approximately occupying a vertex site of the pentagonal face of anido-B-11 cluster. The B(BBN) baron is inserted into a bridging site between B(5) and its crystall ographically imposed mirror equivalent B(10') of the decaborane cluster with B(BBN)B(5)/B(BBN)-B(10') distances of 1.884(4) Angstrom and a B(5)-B(10') distance of 1.823(5) Angstrom. On the other hand the distance of the B(BBN) baron to B(6) and to the mirror equivalent B(9') is an abnormally long 2.142(4) Angstrom, which indicates that the B(BBN) boron is not fully accommodated in the form ation of a nido-[B11H14](-) analogue.