Boronyl Ligand as a Member of the Isoelectronic Series BO− → CO → NO+: Viable Cobalt Carbonyl Boronyl Derivatives?
journal contributionposted on 06.12.2010, 00:00 by Xiaoli Gong, Qian-shu Li, Yaoming Xie, R. Bruce King, Henry F. Schaefer
Recently the first boronyl (oxoboryl) complex [(c-C6H11)3P]2Pt(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 Co2(BO)2(CO)7, with cobalt in the formal d8 +1 oxidation state, are thus isoelectronic with the familiar homoleptic iron carbonyls Fe(CO)5 and Fe2(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 d8 transition metals. Higher energy Co(BO)(CO)n (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 Co2(BO)2(CO)8. However, Co2(BO)2(CO)8 is not viable because of the predicted exothermic loss of CO to give Co2(BO)2(CO)7. The lowest lying Co2(BO)2(CO)7 structure is a triply bridged (2BO + CO) structure closely related to the experimental Fe2(CO)9 structure. However, other relatively low energy Co2(BO)2(CO)7 structures are found, either with a single CO bridge, similar to the experimental Os2(CO)8(μ-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 Co2(BO)2(CO)6 structures are found. The lowest lying triplet Co2(BO)2(CO)6 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 ∼4.3 Å Co···Co distance. The lowest lying singlet Co2(BO)2(CO)6 structures have either two three-electron donor bridging η2-μ-BO groups and no Co···Co bond or one such three-electron donor BO group and a formal Co−Co single bond.