Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM(CO) (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) and BM(CO) (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(CO) complexes are characterized as a sandwich structure composed of two staggered M(CO) fragments and a boron cation, featuring a D symmetry and E 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(CO) complexes involving the filled orbitals of the metals and the empty 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(CO) complexes can be described as a linear conjugated (OC)M═B═M(CO) skeleton with a formal B-M bond index of 1.5. The three delocalized -- 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(CO) cations. As a comparison, the saturated BRh(CO) and BIr(CO) complexes are determined to be a doublet C-symmetry structure with an unbridged (OC)B-M(CO) pattern, involving a two-center σ-type (OC)B → M(CO) 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.