Observation of Transition-Metal-Boron Triple Bonds in IrB2 O- and ReB2 O.

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal dπ →BR back-donation, despite the electron deficiency of boron. An electron-precise metal-boron triple bond was first observed in BiB2 O- [Bi≡B-B≡O]- in which both boron atoms can be viewed as sp-hybridized and the [B-BO]- fragment is isoelectronic to a carbyne (CR). To search for the first electron-precise transition-metal-boron triple-bond species, we have produced IrB2 O- and ReB2 O- and investigated them by photoelectron spectroscopy and quantum-chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB2 O- has a closed-shell bent structure (Cs , 1 A') with BO- coordinated to an Ir≡B unit, (- OB)Ir≡B, whereas ReB2 O- is linear (C∞v , 3 Σ- ) with an electron-precise Re≡B triple bond, [Re≡B-B≡O]- . The results suggest the intriguing possibility of synthesizing compounds with electron-precise M≡B triple bonds analogous to classical carbyne systems.