Abstract Room temperature photolysis of [Cp∗Mo(CO)3Me], 1 with excess of [BH3·THF] led to the isolation of hydrogen-rich arachno-[Cp∗Mo(CO)2B3H8], 2 in good yield. The geometry of arachno-2 consists of a butterfly core similar to that of 7 skeletal electron pairs tetraborane(10). Further, the metal fragment addition reaction of arachno-2 with [Fe2(CO)9] yielded a nido-[Cp∗Mo(CO)2B3H6Fe(CO)3], 3 that plays a pivotal role in bringing a change in the geometry from arachno-2 to nido-3. The reaction of arachno-2 with [Ru3(CO)12], however, yielded known metal carbonyl compound [Cp∗Mo(CO)3]2, 4. All the new compounds have been characterized in solution by mass spectrometry, IR, 1H, 11B, 13C NMR spectroscopy and the solid state X-ray structures of 2 and 3 were unequivocally established by X-ray diffraction analysis. Additionally, we have studied the bonding nature of compounds 2 and 3 with the help of density functional theory (DFT) calculations. © 2014 Elsevier B.V.