\section*{Acknowledgments}

WG would like to thank to A. Michota-Kaminska for making Renishaw inVia Raman system available. We acknowledge support from the Polish National Science Centre (NCN) within the HARMONIA project “HP” (project 2012/06/M/ST5/00344). Z.M. gratefully acknowledges the Slovenian Research Agency (ARRS) for the financial support of the present study within research program P1-0045 Inorganic Chemistry and Technology. The high-field EPR spectra were recorded at the NHMFL, which is funded by the NSF through cooperative agreement no. DMR-1157490 and the State of Florida. S.H. thanks the NSF for financial support (grant DMR-1610226). Research at the University of Warsaw was carried out with the use of CePT infrastructure financed by the European Union – the European Regional Development Fund within the Operational Program "Innovative economy" for 2007-2013 (POIG.02.02.00-14-024/08-00).
The quantum-mechanical calculations performed in Warsaw would not be possible without the ICM supercomputer Okeanos (ADVANCE PLUS, GA67-13). 
J.L. acknowledges support from a CINECA  high-performance computer  project (IsC50\_HTCM) and from Italian MAECI under collaborative Projects SUPERTOP-PGR04879 and  AR17MO7.



\section*{Supplementary materials}
Materials and Methods\\
Figs. S1 to S7\\
Tables S1 to S4\\
References \textit{(31-75)}






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