High-resolution photoelectron imaging of MnB3-: Probing the bonding between the aromatic B3 cluster and 3d transition metals

The B-3 triangular unit is a fundamental bonding motif in all boron compounds and nanostructures. The isolated B-3(-) cluster has a D-3h structure with double sigma and pi aromaticity. Here, we report an investigation of the bonding between a B-3 cluster and a 3d transition metal using high-resolution photoelectron imaging and computational chemistry. Photoelectron spectra of MnB3- are obtained at six different photon energies, revealing rich vibrational information for the ground state detachment transition. The electron affinity of MnB3 is determined to be 1.6756(8) eV, and the most Franck-Condon-active mode observed has a measured frequency of 415(6) cm(-1) due to the Mn-B-3 stretch. Theoretical calculations show that MnB3- has a C-2v planar structure, with Mn coordinated to one side of the triangular B-3 unit. The ground states of MnB3- (B-6(2)) and MnB3 (B-5(2)) are found to have high spin multiplicity with a significant decrease in the Mn-B bond distances in the neutral due to the detachment of an Mn-B-3 anti-bonding electron. The Mn atom is shown to have weak interactions with the B-3 unit, which maintains its double aromaticity with relatively small structural changes from the bare B-3 cluster. The bonding in MnB3 is compared with that in 5d MB3 clusters, where the strong metal-B-3 interactions strongly change the structures and bonding in the B-3 moiety.