Raman and far-IR mode frequency shifts with pressure have been observed under hydrostatic conditions in a gasketed diamond anvil cell (d.a.c.). Using compressibilities calculated from unit cell constants and lattice energies, Grüneisen parameters gammai have been obtained for all observed modes. Where previous data exist for comparison, it is shown that use of a d.a.c. without gaskets (i.e., applying a shear stress to the sample) greatly underestimates the true shifts of IR modes in particular. The Grüneisen parameter gamma1 for a1g nu-tilde 1 of the complex ions is always greater than gamma2. It is shown that this is due to the greater value of anharmonicity of nu-tilde 1, as is the increase in gamma1, gamma2, and gamma3 in the series A2MCl6, where A=Cs, Rb, K. Grüneisen parameters for K2SnCl6 and Rb2TeCl6 are greater than for non-transition metal complexes. Anomalies in the nu-tilde i versus pressure curves for K2SnCl6 and [(CH3)4N]2MCl6 (M=Sn, Te, Pt) are discussed in relation to their structures. Shifts of nu-tilde i with temperature for K2ReCl6 and K2PtCl6 are analyzed into explicit and implicit anharmonic contributions. The Journal of Chemical Physics is copyrighted by The American Institute of Physics.