Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM2(CO)8 + (M = Co, Rh, Ir) Complexes

Investigations of transition-metal boride clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The geometric structures and bonding features of heteronuclear boron-containing transition metal carbonyl cluster cations BM(CO)(6)(+) and BM2(CO)(8)(+) (M = Co, Rh, and Ir) are studied by a combination of the infrared photodissociation spectroscopy and density functional calculations at B3LYP/def2-TZVP level. The completely coordinated BM2(CO)(8)(+) complexes are characterized as a sandwich structure composed of two staggered M(CO)(4) fragments and a boron cation, featuring a D-3d symmetry and E-1(g) electronic ground state as well as metal-anchored carbonyls in an end-on manner. In conjunction with theoretical calculations, multifold metal-boron-metal bonding interactions in BM2(CO)(8)(+) complexes involving the filled d orbitals of the metals and the empty p orbitals of the boron cation were unveiled, namely, one sigma-type M-B-M bond and two pi-type M-B-M bonds. Accordingly, the BM2(CO)(8)(+) complexes can be described as a linear conjugated (OC)(4)M & boxH;B & boxH;M(CO)(4) skeleton with a formal B-M bond index of 1.5. The three delocalized d-p-d covalent bonds render compensation for the electron deficiency of the cationic boron center and endow both metal centers with the favorable 18-electron structure, thus contributing much to the overall structural stability of the BM2(CO)(8)(+) cations. As a comparison, the saturated BRh(CO)(6)(+) and BIr(CO)(6)(+) complexes are determined to be a doublet C-s-symmetry structure with an unbridged (OC)(2)B-M(CO)(4) pattern, involving a two-center sigma-type (OC)(2)B -> M(CO)(4)(+) dative single bond along with a weak covalent B-M half bond. This work offers important insight into the structure and bonding of late transition metal boride carbonyl cluster cations