The reaction of hexahydro-closo-hexaborate(2-) with copper(II) acetate and triphenylphosphine in acetonitrile yields the novel complex, Cu-2[(C6H5)(3)P](4)B6H6. 2CH(3)CN. X-ray structural analysis shows that the complex crystallizes in the triclinic centro-symmetric space group, P (1) over bar, with one molecule per unit cell. Each copper atom bonds to the hexaborate cage through two edge borons, forming a three-center two-electron B-Cu-B bond. The copper in this complex exhibits a trigonal-planar geometry and is a coordinatively unsaturated 16-electron center.