The reaction of the [(η5-C5Me5) MoCl4] complex with [LiBH4·THF] in toluene at -70 °C, followed by pyrolysis at 110 °C, afforded dark brown [(η5-C5Me5Mo)3MoB 9H18], 2, in parallel with the known [(η5- C5Me5Mo)2B5H9], 1. Compound 2 has been characterized in solution by 1H, 11B, and 10C NMR spectroscopy and elemental analysis, and the structural types were unequivocally established by crystallographic studies. The title compound represents a novel class of vertex-fused clusters in which a Mo atom has been fused in a perpendicular fashion between two molybdaborane clusters. Electronic structure calculations employing density functional theory yield geometries in agreement with the structure determinations, and on grounds of density functional theory calculations, we have analyzed the bonding patterns in the structure . © 2010 American Chemical Society.