Three new compounds containing planar, bibridged Cu3X82- (X=Cl- or Br-) anions have been studied. These are (1,1,4-trimethylpiperazinium)Cu3Cl8 (henceforth TMPC), (4-picolinium)2Cu3Cl8 (henceforth 4-PCC), and (4-picolinium)2Cu3Br8 (henceforth 4-PCB). The trimers form stacks parallel to the a axis (TMPC) or the c axis (4-PCC and 4-PCB) through the formation of semicoordinate Cu...X linkages. The powder magnetic susceptibility has been measured for TMPC in fields up to 5000 Oe and at temperatures down to 4.2 K. The individual trimers couple antiferromagnetically with an intratrimer exchange of J/k=-33 K, yielding a spin-1/2 ground state at low temperatures. Intrastack interactions appear to favor a ferromagnetic ground state for the stacks, with antiferromagnetic interactions between stacks. No spin frustration occurs within the stacks, in contrast to previously studied A2Cu3X8 systems. EPR measurements at 9.2 GHz on 4-PCC are consistent with a previous magnetic study that showed that there is antiferromagnetic coupling within each trimer and weak ferromagnetic coupling between trimers. Resolved spectra were observed only at low temperatures, where measurements of the S=1/2 trimer ground state near 15 K in three mutually perpendicular crystal planes gave the principal values gx=2.047±0.001, gy=2.068±0.002, gz=2.264±0.001, where z and x directions are in the ac plane, with the former making an angle of 26±2°with the c axis. Linewidth measurements in the ac plane showed three pairs of maxima and minima with peak-to-peak widths varying in one crystal from 60 to 310 G. Much broader lines were observed in 4-PCB. Unexplained S=3/2 spectra were observed below 30 K in orientations close to the c axis in 4-PCC. © 1990 The American Physical Society.