[1] H G Drickamer, K L Bray, Pressure tuning spectroscopy as a diagnostic for pressure-induced rearrangements (piezochromism) of solid-state copper(II) complexes, Acc. Chem. Res. 23, 55 (1990).

[2] J Ferguson, Electronic absorption spectrum and structure of CuCl4=, J. Chem. Phys. 40, 3406 (1964).

[3] L Nataf, F Aguado, I Hernandez, R Valiente, J Gonzalez, M N Sanz-Ortiz, H Wilhelm, A P Jephcoat, F Baudelet, F. Rodriguez. Volume and pressure dependences of the electronic, vibrational, and crystal structures of C s 2 CoC l 4: Identification of a pressure-induced piezochromic phase at high pressure, Phys. Rev. B 95, 014110 (2017).

[4] Y Xu, S Carlson, K S oderberg, R Norrestam, High-pressure studies of Cs2CuCl4 and Cs2CoCl4 by X-ray diffraction methods, J. Sol. St. Chem. 153, 212 (2000).

[5] H B Yi, F F Xia, Q Zhou, D Zeng, [CuCl3]- and [CuCl4] 2- hydrates in concentrated aqueous solution: A density functional theory and ab initio study, J. Phys. Chem. A 115, 4416 (2011).

[6] R Valiente, F Rodriguez, Effects of chemical pressure on the charge-transfer spectra of CuX4^{2-}-complexes formed in Cu^{2+}-doped (M= Zn, Mn, Cd, Hg; X= Cl, Br), J. Phys.: Condens. Matter, 10, 9525 (1998).

[7] R Valiente, F Rodriguez, Thermochromic properties of the ferroelectric Cu2+-doped [(CH3)4N]HgBr3: Study of the temperature-induced dichroism, J. Phys.: Condens. Matter 11, 2595 (1999).

[8] S Klotz, J C Chervin, P Munsch, G Le Marchand, Hydrostatic limits of 11 pressure transmitting media, J. Phys. D: Appl. Phys. 42, 075413 (2009).

[9] A Moral, F Rodriguez, New double beam spectrophotometer for microsamples. Application to hydrostatic pressure experiments, Rev. Scient. Inst. 66, 5178 (1995).

[10] K Syassen, Ruby under pressure, High Pressure Res. 28, 75 (2008).

[11] J A Barreda-Argueso, F Rodriguez, Microscopio para la caracterizacion espectroscopica de una muestra, Spanish Patent No. ES 2 461 015 A1 (2014).

[12] J S Griffith, The theory of transition-metal ions, Cambridge Univ. Press (1980).

[13] M Atanasov, B Delley, D Reinen, A DFT study of the energetical and structural landscape of the tetrahedral to square-planar conversion of Tetrahalide complexes of Copper(II), Z. Anorg. Allg. Chem. 636, 1740 (2010).

[14] F Rodriguez, D Hernandez, J Garcia-Jaca, H Ehrenberg, H Weitzel, Optical study of the piezochromic transition in CuMoO 4 by pressure spectroscopy, Phys. Rev. B. 61, 16497 (2000).

[15] M A Hitchman, The influence of vibronic coupling on the spectroscopic properties and stereochemistry of simple 4- and 6-coordinate copper(II) complexes, Comments Inorg. Chem. 15, 197 (1994).

[16] R G McDonald, M J Riley, M A Hitchman, Angular overlap treatment of the variation of the intensities and energies of the dd transitions of the tetrachlorocuprate(2-) ion on distortion from a planar toward a tetrahedral geometry: interpretation of the electronic spectra of bis(N-benzylpiperazinium) tetrachlorocuprate(II) bis(hydrochloride) and N-(2-ammonioethyl) morpholinium tetra- chlorocuprate(II), Inorg. Chem. 27, 894 (1988).

[17] M Atanasov, D Ganyushin, K Sivalingam, F Neese, A modern first-principles view on ligand field theory through the eyes of correlated multireference wavefunctions, Struct. Bond 143, 149 (2012).