Fig. Structure of the anionic [V2] dimer in 1. All reported [V2] dimers are structurally similar, differing only in the presence of either an O 2-or NR 2-ion on the sixth coordination site on each V centre. Complexes 1, 3-6 all contain the calix [8]arene in the hepta-deprotonated H1TBC [8] form and the complexes are charge balanced by a single cation. Complex 2 contains H2TBC [8] and is neutral. Colour code: V = dark green, O = red, C = grey. H atoms and counter ions omitted for clarity.