A new decaoxidooctaborate(2-) anion, [B8O10(OH)6](2-): synthesis and characterization of [Co(en)3][B5O6(OH)4][B8O10(OH)6)]·5H2O (en = 1,2-diaminoethane).

The synthesis and X-ray diffraction structure of [Co(en)3][B5O6(OH)4][B8O10(OH)6]·5H2O (1) are reported. Compound 1 arises through a selective-templating process from a Dynamic Combinatorial Library of polyborate anions. Compound 1 contains two different polyborate species, with [B8O10(OH)6](2-) being particularly novel. It is comprised of fused tetraborate and pentaborate anions with a 4-coordinate B atom and a 3-coordinate O atom in common.