Since the discovery of the first drum-like CoB16-
complex, metal-doped drum-like boron nanotubular structures have been investigated with various metal dopants and different tubular size, forming a new class of novel nanostructures. The CoB16-
cluster was found to be composed of a central Co atom coordinated by two fused B8
rings in a tubular structure, representing the potential embryo of metal-filled boron nanotubes and providing opportunities to design one-dimensional metal-boron nanostructures. Here we report improved photoelectron spectroscopy and a more in-depth electronic structure analysis of CoB16-
, providing further insight into the chemical bonding and stability of the drum-like doped boron tubular structures. Most interestingly, we find that the central Co atom has an unusually low oxidation state of ?1 and neutral CoB16
can be viewed as a charge transfer complex (Co-
), suggesting both covalent and electrostatic interactions between the dopant and the boron drum.