Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM 2 (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 BM 2 (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 BM 2 (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 1 E 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 BM 2 (CO) 8 + complexes involving the filled d orbitals of the metals and the empty p orbitals of the boron cation were unveiled, namely, one σ-type M-B-M bond and two π-type M-B-M bonds. Accordingly, the BM 2 (CO) 8 + complexes can be described as a linear conjugated (OC) 4 M═B═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 BM 2 (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 σ-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.