  Solvent-solute interactions in precursor solutions of lead halide perovskites
(LHP) critically impact the quality of solution-processed materials, as they
lead to the formation of a variety of poly-iodoplumbates that act as building
blocks for LHP. The formation of [PbI$_{2+n}$]$^{n-}$ complexes is often
expected in diluted solutions while coordination occurring at high
concentrations is not well understood yet. In a combined \textit{ab initio} and
experimental work, we demonstrate that the optical spectra of the
quasi-one-dimensional iodoplumbate complexes PbI$_2$(DMSO)$_4$,
Pb$_2$I$_4$(DMSO)$_6$, and Pb$_3$I$_6$(DMSO)$_8$ formed in dimethyl sulfoxide
solutions are compatible with the spectral fingerprints measured at high
concentrations of lead iodide. This finding suggests that the formation of
polynuclear lead-halide complexes should be accounted for in the interpretation
of optical spectra of LHP precursor solutions.
