High-Resolution Photoelectron Imaging of IrB3 - : Observation of a π-Aromatic B3 + Ring Coordinated to a Transition Metal.

In a high-resolution photoelectron imaging and theoretical study of the IrB3 - cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B3 ring coordinated with the Ir atom. The isomer with the higher EA consists of a B3 ring with a bridge-bonded Ir atom (Cs , 2 A'), and the second isomer features a tetrahedral structure (C3v , 2 A1 ). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C3v isomer involves significant covalent Ir-B bonding and weak ionic bonding with charge transfer from B3 to Ir, and can be viewed as an Ir-(η3 -B3 + ) complex. This study provides the first example of a boron-to-metal charge-transfer complex and evidence of a π-aromatic B3 + ring coordinated to a transition metal.