Cu(hfacac)(2)(NH3) (hfacac = CF3C(O)CHC(O)CF3-) is shown to have a trigonal-bipyramidal structure with Cu-O (axial) = 1.945(3) Angstrom, Cu-O (equatorial) = 2.075(3) Angstrom and Cu-N = 1.933(6) Angstrom. This contrasts with the square-pyramidal structure of Cu(hfacac)(2)(H2O), which has a long (2.204(3) Angstrom) Cu-OH2 bond. The ammine complex retains its NH3 ligand upon vacuum sublimation, while the H2O complex loses water. The difference in coordination geometry and Cu-L distance (L = H2O, NH3) is traced to minimizing a antibonding interactions with the stronger ligand L, which, by the criterion of 10Dq, is NH3. Crystal data (-67 degrees C) : a = 20.594(6) Angstrom, b = 8.881(2) Angstrom, c = 8.619(2) Angstrom, beta = 104.49(1)degrees with Z = 4 in space group C2/c.