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 ¹E_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 σ-type M–B–M bond and two π-type M–B–M bonds. Accordingly, the BM₂(CO)₈⁺ complexes can be described as a linear conjugated (OC)₄M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 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)₂B–M­(CO)₄ pattern, involving a two-center σ-type (OC)₂B → 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.