  Polonium (Po, Z = 84) is a main-block element with poorly known
physico-chemical properties. Not much information has been firmly acquired
since its discovery by Marie and Pierre Curie in 1898, especially regarding its
speciation in aqueous solution and spectroscopy. In this work, we revisit the
absorption properties of two complexes, [PoCl5]^- and [PoCl6]^{2-}, using
quantum mechanical calculations. These complexes have the potential to exhibit
a maximum absorption at 418 nm in HCl medium (for 0.5 mol/L concentrations and
above). Initially, we examine the electronic spectra of the Po^{4+} free ion
and of its isoelectronic analogue, Bi^{3+}. In the spin-orbit configuration
interaction (SOCI) framework. Our findings demonstrate that the SOCI matrix
should be dressed with correlated electronic energies and that the quality of
the spectra is largely improved by decontracting the reference states at the
complete active space plus singles (CAS+S) level. Subsequently, we investigate
the absorption properties of the [PoCl5]^- and [PoCl6]^{2-} complexes in two
stages. Firstly, we perform methodological tests at the MP2/def2-TZVP gas phase
geometries, indicating that the decontraction of the reference states can there
be skipped without compromising the accuracy significantly. Secondly, we study
the solution absorption properties by means of single-point calculations
performed at the solvated geometries, obtained by an implicit solvation
treatment or a combination of implicit and explicit solvation. Our results
highlight the importance of saturating the first coordination sphere of the
Po^{IV} ion to obtain a qualitatively correct picture. Finally, we conclude
that the known-for-decades 418 nm peak could be attributed to a mixture of both
the [PoCl5(H2O)]^- and [PoCl6]^{2-} complexes. This finding not only aligns
with the behaviour of the analogous Bi^{III} ion under similar conditions
but...
