This work investigates the electronic and vibrational structures of the pseudo Jahn-Teller (PJT) compound ${\mathrm{Rb}}_2{\mathrm{CuCl}}_4{\left({\mathrm{H}}_2\mathrm{O}\right)}_2$, along with their pressure dependence using optical absorption and Raman spectroscopy. The rhombic $\left({\mathrm{D}}_{2\mathrm{h}}\right)$ local structure of the ${\mathrm{CuCl}}_4{\left({\mathrm{H}}_2\mathrm{O}\right)}_2^{2-}$ complex undergoes a progressive pressure-induced transformation, in which the two short and two long Cu-Cl bonds (perpendicular to the ${\mathrm{H}}_2\mathrm{O}\text{−}\mathrm{Cu}\text{−}{\mathrm{OH}}_2$ axis) converge in length, ultimately suppressing the PJT distortion at 10 GPa. This yields a tetragonal $\left({\mathrm{D}}_{4\mathrm{h}}\right)$ symmetry with four equivalent Cu-Cl bonds. The spectra reveal the crystal-field electronic structure of the complex in both ${\mathrm{D}}_{4\mathrm{h}}$ and PJT-distorted ${\mathrm{D}}_{2\mathrm{h}}$ geometries. Their energies, calculated via the angular overlap model (AOM), accurately describe the ground state and the excited states. Notably, the first crystal-field transition—between PJT-active levels ${\mathrm{a}}_{1\mathrm{g}}(3z^2-r^2)$ and ${\mathrm{b}}_{1\mathrm{g}}(x^2-y^2)$ of ${\mathrm{Cu}}^{2+}$—occurs at 1.32 eV in the PJT-distorted $\left({\mathrm{D}}_{2\mathrm{h}}\right)$ phase but shifts to 1.09 eV in the undistorted ${\mathrm{D}}_{4\mathrm{h}}$ phase, directly quantifying the PJT-induced splitting. The vibrational modes exhibit unusual pressure dependence: The frequency associated with the long Cu-Cl bond in ${\mathrm{D}}_{2\mathrm{h}}$ shows a large pressure shift, while the short Cu-Cl bond mode shifts slightly. These converge at the transition pressure ($P_t$ = 10 GPa), confirming the progressive suppression of the PJT distortion (from $Q_{{\mathrm{b}}_{1\mathrm{g}}}^0=0.7$ Å at ambient pressure to $Q_{{\mathrm{b}}_{1\mathrm{g}}}^0=0$ at $P_t$). A Grüneisen-type model correlates the pressure-dependent Cu-Cl stretching frequencies with bond-distance variations, providing an alternative method to track PJT distortion under pressure. The local volume $V\left(P\right)$ of the complex, derived from this approach, follows a Murnaghan equation of state with a bulk modulus $B_{\mathrm{loc}}=22\left(2\right)$ GPa, consistent with the x-ray diffraction value $(B_{\mathrm{cryst}}=20.5 \mathrm{GPa})$.

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• Electronic structure
• High-pressure studies
• Pseudo Jahn-Teller effect
• Structural phase transition
• Vibrational states
• Layered crystals
• Metal halides
• Single crystal materials
• Cluster models
• Optical spectroscopy
• Symmetries
• X-ray diffraction