A series of metal-rich metallaboranes of groups 7 and 9 comprising triply bridging borylene and boride units have been isolated and structurally characterized. Thermolysis of nido-[(RhCp∗)2B3H7] (1; Cp∗ = η5-C5Me5) with [Co2(CO)8] led to the isolation of tetrametallic [(μ3-BH)(RhCp∗)2(μ-CO)(μ3-CO)Co2(CO)4] (2), featuring a triply bridging borylene unit, and the trimetallic cluster [(μ3-BH)(μ-H)(RhCp∗)2(μ-CO)3Co(CO)] (3) that contains a triply bridging hydrido(borylene) unit. The borylene BH unit of 2 is coordinated to a deltahedral face of a tetrametallic tetrahedron in a μ3 fashion. Cluster 3 is a rare example of a tetrahedral metallaborane featuring a hydrido(borylene) unit. In an attempt to synthesize the Mn analogues of 2 and 3, a similar reaction was carried out with [Mn2(CO)10] that afforded the trimetallic cluster [(μ3-BH)(RhCp∗)2(μ-CO)3MnH(CO)3] (5) having a triply bridging borylene moiety, the two heterometallic μ9-boride clusters [(RhCp∗)3Rh(CO)3(μ-CO)3MnH(CO)3B3H2] (6) and [(RhCp∗)3Mn(CO)32Rh(CO)2B4H3] (7) and the unusual tetrametallic complex [(RhCp∗)2(μ-CO)2(μ3-η3-CO2)Mn2(CO)9] (8). Clusters 6 and 7 are both unusual heterometallic metal-rich boride clusters, where the boride boron atom is encapsulated inside a tricapped trigonal prism depicting a μ9-bonding mode. Compound 8 is a unique example of a metal carbonyl compound in which a CO2 group is bridging two Rh atoms and one Mn atom in a μ3-η3 fashion. To explore this chemistry with a heavier transition metal, we have carried out the thermolysis of arachno-[IrCp∗H2(B3H7)] (9) with [Mn2(CO)10], which afforded the face-fused iridaborane cluster [(IrCp∗)3Ir(CO)23(μ-CO)(μ3-CO)B] (10). Compound 10 can also be viewed as a boride cluster, where the naked boron is coordinated to iridium centers in a unique μ5 coordination mode. All of the compounds have been characterized by 1H, 11B, and 13C NMR spectroscopy and mass spectrometry, and the structures of 2, 3, 5, 6, 8, and 10 have been unambiguously established by crystallographic analyses. Computational studies show that a substantial amount of overlap occurs between the metal frameworks and borylene/boride units. © 2021 American Chemical Society.