Prompted by the previous report of BFe(CO)3− possessing a B≣Fe quadruple bond, the detailed potential energy surface exploration for the BMC3O3− (M = Fe, Ru, Os) formulation reveals that the most stable isomer for M = Ru, Os has a C s-symmetric CM(CO)2(BO)− (M = Ru, Os) structure in a singlet electronic state with an ultra-short C–M bond along the center axis, whereas for M = Fe, the global minimum is a C s-symmetric isomer in the triplet electronic state where C of (OC)C(BO) binds with Fe of the FeCO unit. BM(CO)3− is a kinetically stable high-lying isomer for all cases. Detailed bonding analyses on CM(CO)2(BO)− (M = Ru, Os) reveal that the C–M bond can be described as a quadruple bond consisting of a strong electron-sharing C–M(CO)3− σ and π bonds, accompanied by a strong C←M(CO)3− π bond and a weak C→M(CO)3− σ bond. These bonding motifs expand the landscape of high-order multiple bonding between main-group elements and transition metals, particularly in the context of heavier transition-metal carbonyl complexes.