Preference of Three-Electron Donor Boronyl Groups over Metal-Metal Multiple Bonding in Unsaturated Binuclear Manganese Carbonyl Boronyls: Comparison with Isoelectronic Binuclear Chromium Carbonyls

Abstract The lowest energy structures for the unsaturated Mn 2 (BO) 2 (CO) n ( n  = 8, 7) are very different from those of the isoelectronic Cr 2 (CO) n +2 owing to the greater basicity of the oxygen atoms in BO groups relative to CO groups. Thus the lowest energy Mn 2 (BO) 2 (CO) n ( n  = 8, 7) structures have two bridging three-electron donor η 2 µ BO groups bonded to the manganese atoms through both their boron and oxygen atoms. The long Mn⋯Mn distances in these structures indicate the lack of metal–metal bonding. These Mn 2 (BO) 2 (CO) n ( n  = 8, 7) structures thus differ from their Cr 2 (CO) n +2 analogues, which have normal two-electron donor CO groups and Cr Cr bonds. The only example of manganese-manganese multiple bonding in the Mn 2 (BO) 2 (CO) n systems occurs in a high-energy triply bridged Mn 2 (BO) 2 (CO) 7 structure with a short ∼2.3 A Mn Mn distance suggesting a formal triple bond. This structure is completely analogous to the lowest energy Cr 2 (CO) 9 structure.