CHARACTERIZATION OF TRIGONAL BIPYRAMIDAL [CUCL3(L)(2)](-) ANIONS - EVIDENCE FOR THE ABSENCE OF A PSEUDO-JAHN-TELLER EFFECT

The structure of the [CuCl3(L)(2)](-) anions (L = H2O, NH3) has been i nvestigated in two environments: in the orthorhombic crystal (dabcoH(2 ))(2)Cl-3[CuCl3(H2O)(2)] . H2O and doped into the rhombohedral A(dabco H(2))Cl-3 lattice. In the former, the anion exhibits small distortions from trigonal bipyramidal geometry due to lattice strains (hydrogen b onding). In the latter, the [CuCl3(L)(2)](-) anions occupy sites of D- 3 symmetry. EPR studies confirm that the trigonal bipyramidal structur e is retained down to 4.2 K. Angular-overlap method calculations indic ate the existence of a single minimum in the potential surface corresp onding to trigonal bipyramidal geometry. A softening of the potential in the directions of C-2v distortions is observed. The stability of th e trigonal bipyramidal geometry, in contrast to the instability of thi s geometry in the CuCl53- anion, is attributed to the destabilization of the excited E-2' state by the stronger bonding in the axial positio ns of the trigonal bipyramid.