Recent research into the transition-metal chemistry of boron has unveiled a remarkable ability of boron-based ligands to adopt a wide diversity of different coordination modes ( Figure 1). Classifications have been made by the number of substituents at boron and by the number of transition metals attached to the boron center. [1] Experimental and theoretical studies clearly identified a classical bonding situation with two-center-two-electron (2c2e) bonds for most borane (I) and boryl (II) complexes. By contrast, borders between electron-precise and non-classical structures tend to become blurred moving to borylene (III-V) or boride (VI) ligands.Various synthetic strategies have enabled the isolation of terminal (mono-) and m 2 /m 3 bridging (di-and trinuclear) borylene complexes, which usually require kinetic stabilization by sterically demanding ligands at the transition-metal center, or by bulky and/or electron-donating substituents at boron. [2] Trinuclear m 3 bridging borylene complexes still remain rather exotic species, and only few examples have been structurally authenticated so far. Thus, Fehlner and Rheingold reported on the synthesis of a phosphacobaltaborane (1; Scheme 1) featuring a m 3 -BPh ligand. [3] Later on, Suzuki et al. succeeded in the generation of the trinuclear ruthenium species 2 (R = H, CN), albeit without structural authentication by X-ray diffraction. [4] Reaction of the B-H derivative with protic reagents (MeOH, EtOH) resulted in the loss of H 2 and formation of the corresponding alkoxyborylenes 2 (R = OMe, OEt), which implies a hydridic nature of the BÀH moiety. Consequently, homotrimetallic borylenes 2 might also be regarded as trimetalloborate complexes, thus emphasizing the difficulties associated with the exact description of such uncommon species. However, the borylene ligands in 1 and 2 always act as two-electron donors within a non-classical cluster structure. Therefore, an electronprecise bonding situation is only amenable in the absence of any covalently bound exohedral ligand at the boron center of the M 3 B core.At best, this is achieved by formal removal of the B-bound substituent R resulting in trinuclear m 3 boride complexes in which boron is exclusively coordinated to transition-metal fragments. Several examples for this family of compounds have been realized to date, featuring varying structures (Tshaped, Y-shaped, trigonal) with variable degrees of metalboron bonding. [1, 5] However, the transition from m 3 borylene to m 3 boride complexes involves rehybridization of the boron center from sp 3 to either sp (T-shaped, Y-shaped) or sp 2 (trigonal) and loss of the tetrahedral M 3 B structure. For instance, trinuclear 3, which closely approaches an ideal trimetalloboride structure with an electron-precise bonding situation, is essentially trigonal-planar (sum of angles: 359.38). [5e] These results clearly imply that saturation of the fourth valence at boron by a non-covalent interaction is required to make a tetrahedral, electron-precise m 3 boride complex feasible...