Recently the first boronyl (oxoboryl) complex [(c-C6H11)(3)P](2)Pt(BO)Br was synthesized. The boronyl ligand in this complex is a member of the isoelectronic series BO -> CO -> NO+. The cobalt carbonyl boronyls Co(BO)(CO)(4) and Co-2(BO)(2)(CO)(7), with cobalt in the formal d(8) +1 oxidation state, are thus isoelectronic with the familiar homoleptic iron carbonyls Fe(CO)(5) and Fe-2(CO)(9). Density functional theory predicts Co(BO)(CO)(4) to have a trigonal bipyramidal structure with the BO group in an axial position. The tricarbonyl Co(BO)(CO)(3) is predicted to have a distorted square planar structure, similar to those of other 16-electron complexes of d(8) transition metals. Higher energy Co(BO)(CO), (n = 3, 2) structures may be derived by removal of one (for n = 3) or two (for n = 2) CO groups from a trigonal bipyramidal Co(BO)(CO)(4) structure. Structures with a CO group bridging 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units and no Co Co bond are found for Co-2(BO)(2)(CO)(9). However, Co-2(BO)(2)(CO)(5) is not viable because of the predicted exothermic loss of CO to give Co-2(BO)(2)(CO)(7). The lowest lying Co-2(BO)(2)(CO)(7) structure is a triply bridged (2BO + CO) structure closely related to the experimental Fe-2(CO)(9) structure. However, other relatively low energy Co-2(BO)(2)(CO)(7) structures are found, either with a single CO bridge, similar to the experimental Os-2(CO)(8)(mu-CO) structure; or with 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units joined by a single Co-Co bond with or without semibridging carbonyl groups. Both triplet and singlet Co-2(BO)(2)(CO)(6) structures are found. The lowest lying triplet Co-2(BO)(2)(CO)(5) structures have a Co(CO)(3)(BO)(2) unit coordinated to a Co(CO)(3) unit through the oxygen atoms of the boronyl groups with a non-bonding similar to 4.3 angstrom Co ... Co distance. The lowest lying singlet Co-2(BO)(2)(CO)(6) structures have either two three-electron donor bridging eta(2)-mu-BO groups and no Co ... Co bond or one such three-electron donor BO group and a formal Co-Co single bond.