Irradiation of [Re2(CO)10] in the presence of BH3·thf resulted in the formation of several rhenium diborane(6) species, for example, [(OC)4ReRe(CO)32(μ3-η2:η2:η2-B2H6)(μ-H)], 1; [(OC)4Re2Re(CO)3(μ3-η2:η2:η1-B2H6)(μ-H)], 2; and [(OC)4Re2(μ-η2:η2-B2H6)], 3, comprising diverse coordination modes of the [B2H6]2- ligand. Compound 1 contains a tris(bidentate) [B2H6]2- unit, whereas 2 consists of an unsymmetrically bound [μ3-η2:η2:η1-B2H6]2- ligand. In contrast, the irradiation of [Mn2(CO)10] with BH3·thf yielded only the Mn analogue of 1, [(OC)4MnMn(CO)32(μ3-η2:η2:η2-B2H6)(μ-H)], 1′. In an attempt to generate the bimetallic Mn-diborane(6), we have carried out the reaction of 1′ with PCy3 under photolytic conditions. The reaction led to the formation of two single base stabilized unsymmetrical diborane(5) species, [Mn(CO)3Mn(CO)2PCy3(μ-η2:η2-B2H5·PCy3)(μ-H)], 4, and [Mn(CO)2PCy3(η3-B2H5·PCy3)], 5. As [B2H6]2- and [B2H5·PCy3]- are isoelectronic, the bondings in 4 and 5 are analogous to that of diborane(6) species 1-3. All the new species have been characterized spectroscopically, and their structures were further confirmed by single-crystal X-ray diffraction studies. DFT-type quantum chemical calculations were carried out that provided insight into the bonding interaction of [B2H6]2- and [B2H5·PCy3]- with the M(CO)n fragments. Copyright © 2020 American Chemical Society.