Thermolysis of mono metal carbonyl fragment, [M′(CO) 5 ·thf, M′ = Mo and W, thf = tetrahydrofuran] with an in situ generated intermediate, obtained from the reaction of [Cp∗MCl 4 ] (M = Mo and W, Cp∗ = 1,2,3,4,5-pentamethylcyclopentadienyl) with [LiBH 4 ·thf], yielded dimetallaboranes, 1 and 2. Isolations of [{Cp∗M(CO)} 2 B 4 H 6 ] (M = Mo (1) and W(2)) provide direct evidence for the existence of saturated molybdaborane and tungstaborane clusters, [(Cp∗M) 2 B 4 H 10 ]. Our extensive theoretical studies together with the experimental observation suggests that the intermediate may be a saturated cluster [(Cp # M) 2 B 4 H 10 ], not unsaturated [(Cp # M) 2 B 4 H 8 ] (Cp # = Cp or Cp∗), which was proposed earlier by Fehlner. Furthermore, in order to concrete our findings, we isolated and structurally characterized analogous clusters [(Cp∗Mo) 2 (CO)(μ-Cl)B 3 H 4 W(CO) 4 ] (3) and [(Cp∗WCO) 2 (μ-H) 2 B 3 H 3 W(CO) 4 ] (4). All the compounds have been characterized by solution-state 1 H, 11 B, IR, and 13 C NMR spectroscopy, mass spectrometry, and the structural architectures of 1, 3, and 4 were unequivocally established by X-ray crystallographic analysis. The density functional theory calculations yielded geometries that are in close agreement with the observed structures. Both the Fenske-Hall and Kohn-Sham molecular orbital analyses showed an increased thermodynamic stability for [(Cp # M) 2 B 4 H 10 ] compared to [(Cp # M) 2 B 4 H 8 ]. Furthermore, large HOMO-LUMO gap and significant cross cluster M-M bonding have been observed for clusters 1-4. © 2018 American Chemical Society.