The targeted high-yield syntheses of a number of new clusters by the controlled addition of BH, Fe(CO)(3), or Co(CO)(3) fragments to dinuclear group 6 metallaboranes are reported. The reaction of the chromaborane, (Cp*Cr)(2)B4H8 (1), Cp* = eta(5)-C5Me5, with BHCl2 . SMe2 results in a cluster expansion reaction giving the green-brown diamagnetic species (Cp*Cr)(2)B5H9 (2) in 43% yield. The six skeletal electron pair (sep) cluster has a structure based on a trigonal bipyramidal Cr2B3 unit with adjacent Cr2B faces capped by BH3 fragments. Reaction of 1 with the dinuclear metal carbonyls Fe-2(CO)(9) and Co-2(CO)(8) leads to the isolation of the mixed-metal metallaboranes (Cp*Cr)(2)B4H8Fe(CO)(3) (3) and (Cp*Cr)(2)B4H7Co(CO)(3) (4) in yields of 76 and 87%, respectively. 3 and 4 can also be viewed as six-sep bicapped trigonal bipyramidal clusters with the metal carbonyl occupying one of the capping positions. Alternatively, they can be viewed as complexes between M(CO)(3) transition metal fragments (M = Fe, Co) and the four-electron (Cp*Cr)(2)B4H8 (M = Fe) or three-electron (Cp*Cr)(2)B4H7 (M = Co) ligands. Ln this way, by forming two M-Cr bonds and utilizing either one (M = Co) or two (M = Fe) BH-->M donor-acceptor bonds, each metal attains an 18-electron configuration. At elevated temperatures (ca. 80 degrees C), 3 is fluxional, with the Fe(CO)(3) fragment "swinging" between equivalent pairs of BH2 donor groups. Reaction of the molybdaborane (Cp*Mo)(2)B5H9 (5) with Fe-2(CO)(9) gives the orange mixed-metal cluster (Cp*Mo)(2)B5H9Fe(CO)(3) (6) in 48% yield. 6 has a bicapped octahedral geometry with the Fe(CO)(3) fragment occupying one of the high-connectivity cluster vertexes, rather than a capping position, That 5 undergoes the expected geometric changes (bicapped trigonal bipyramid --> bicapped octahedron) on addition of the two-electron Fe(CO)(3) fragment (giving 6) contrasts with the minimal structural perturbation on addition of the same fragment to 1 (yielding 3). This in turn provides further evidence for the description of 1 as an "electronically unsaturated" cluster.