Irradiation of [Re-2(CO)(10)] in the presence of BH3 center dot thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)(4)Re}{Re(CO)(3)}(2) (mu(3)-eta(2):eta(2):eta(2)-B2H6)(mu-H)], 1; [{(OC)(4)Re}(2) {Re(CO)(3)}(mu(3)-eta(2):eta(2):eta(1)-B2H6)(p-H)], 2; and [{(OC)(4)Re}(2) (mu-eta(2):eta(2)-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 [mu(3)-eta(2): eta(2):eta(1)-B2H6](2-) ligand. In contrast, the irradiation of [Mn-2(CO)(10)] with BH3 center dot thf yielded only the Mn analogue of 1, [{(OC)(4)Mn}{Mn(CO)(3)}(2) (mu(3)-eta(2):eta(2):eta(2)-B2H6)(mu-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)(3)}{Mn(CO)(2)PCy3}(mu-eta(2):eta(2)-B2H5 center dot PCy3)(mu-H)], 4, and [{Mn(CO)(2) PCy3}(eta(3)-B2H5 center dot PCy3)], 5. As [B2H6](2-) and [B2H5 center dot 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 center dot PCy3](-) with the M(CO)(n) fragments.