Cyclopentadienyl (Cp) ligands have received considerable attention mainly because of their pi six-electron donation capability. Tris(pyrazolyl)borate ligands (Tp) are often compared with Cp because of their identical charge, number of donated electrons, and similar facial coordinating geometry. Their six-electron donation, however, is formally sigma-type. The X-ray structure of the [Tp(3)(3)(CF)(,CH)CuK(mu (4)-CO3)KTp(3)(3)(CF)(,CH)](2) aggregate, 1, reveals for the first time an unprecedented eta (5)-Tp(3)(3)(CF)(,CH) potassium bonding interaction. I crystallizes in the triclinic space group P (1) over bar with a = 12.0411(2) Angstrom, b = 14.9791(2) Angstrom, c = 16.0567(3) Angstrom, alpha = 71.301(1)degrees, beta = 69.785(1)degrees, gamma = 66.539(1)degrees, and Z = 2. Both K-F and K-mu (4)-CO32-interactions stabilize the aggregate, as suggested by the lack of hexanuclear aggregation and K incorporation in the absence of fluorine groups or when O=CO22- is replaced by CH3-CO2-. In the latter case we have isolated the complex [CuTp(3)(3)(CF)(,CH)(CH(3)CO(0)2)], 2, which retains a Cu coordination sphere similar to that encountered in the Tp(3)(3)(CF)(,CH)Cu subset of 1. The mononuclear complex 2 crystallizes in the monoclinic space group P2(1)/c with a = 14.1474(2) Angstrom, b = 14.1474(2) Angstrom, c = 19.0456(6) Angstrom, beta = 99.012(2)degrees, and Z = 4. The novel eta (5)-coordination mode revealed in 1 suggests that Tp ligands might function not only as a donors but also as Cp-like pi donors. The eta (5)-coordination mode might therefore constitute a new potential common denominator of these two important classes of ligands.