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The synthesis and structural characterization of a series of heterotrimetallic ruthenaborane clusters are reported. The photolytic reaction of <i>nido</i>-[(Cp*Ru)₂(<i>µ</i>-H)₂B₃H₇] (<i>nido</i>-<b>1</b>) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) with [M(CO)₅·THF] (THF = tetrahydrofuran, M = Mo and W) yielded the heterotrimetallic clusters <i>pileo</i>-[(Cp*Ru)₂{M(CO)₃}(<i>µ</i>-CO)(<i>µ</i>-H)(<i>µ</i>₃-BH)B₂H₅], M = Mo (<b>2</b>), W (<b>3</b>) and the known <i>arachno</i> ruthenaboranes [1,2-(Cp*Ru)(Cp*RuCO)(<i>µ</i>-H)B₃H₈] (<b>I</b>) and [{Cp*Ru(CO)}₂B₂H₆] (<b>II</b>). In an attempt to synthesize the Mn-analog of <b>2</b> and <b>3</b>, we performed a similar reaction of <i>nido</i>-<b>1</b> with [Mn₂(CO)₁₀], which afforded the heterotrimetallic <i>pileo</i>-[(Cp*Ru){Mn(CO)₃}(<i>µ</i>-H)₂(<i>µ</i>₃-BH)B₂H₅] (<b>4</b>) cluster along with the reported trimetallic hydrido(hydroborylene) species [(Cp*Ru)₂{Mn(CO)₃}(<i>µ</i>-H)(<i>µ</i>-CO)₃(<i>µ</i>-BH)] (<b>III</b>). Ruthenaboranes <b>2</b>, <b>3</b> and <b>4</b> are isoelectronic and isostructural. The geometry of <b>2</b>–<b>4</b> can be viewed as a triangle face-fused square pyramidal and tetrahedral geometry, in which the apical vertex of the tetrahedron is occupied by a <i>µ</i>₃–BH moiety. All of these <i>pileo</i> ruthenaborane clusters obey Mingos’ fusion formalism. Clusters <b>2</b>–<b>4</b> were characterized using multinuclear NMR, IR spectroscopies and electrospray ionization mass spectrometry. The single-crystal X-ray diffraction studies of clusters <b>2</b> and <b>4</b> confirmed their structures. Further, density functional theory (DFT) studies of these <i>pileo</i> ruthenaboranes have been carried out to investigate the nature of bonding, fusion and electronic structures.