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I have read that sulfur dioxide acts as both Lewis base (due to presence of lone pair over $\ce{S}$ and/or $\ce{O})$ and Lewis acid (due to presence of vacant d orbital). I found many examples of $\ce{SO2}$ acting as a Lewis acid (towards $\ce{NMe3},$ $\ce{H2O},$ etc.), but could not find any for its Lewis base behavior. Could someone provide some reactions/equations, please?

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You generally want to find a much stronger Lewis acid (typically forming a solvate/adduct) or make a complex with a transition metal: this way either unshared electron pair of oxygen or sulfur can be donated. Canonical examples are given in Shriver & Atkins’ Inorganic Chemistry [1, p. 135]:

To act as a Lewis base, the $\ce{SO2}$ molecule can donate either its $\ce{S}$ or its $\ce{O}$ lone pair to a Lewis acid. When $\ce{SbF5}$ is the acid, the $\ce{O}$ atom of $\ce{SO2}$ acts as the electron pair donor, but when $\ce{Ru(II)}$ is the acid, the $\ce{S}$ atom acts as the donor … $(\ce{[RuCl(NH3)4(SO2)]+}).$

SbF5-SO2$\qquad$RuCl(NH3)4(SO2)]+

Figure 1. Left: Crystal structure of $\ce{SbF5 · SO2}$ (ICSD-67273 [2]). Right: Crystal structure of $\ce{[RuCl(NH3)4(SO2)]+}$ (ICSD-26061 [3]). Color code: $\color{#EEEEEE}{\Large\bullet}~\ce{H}$; $\color{#3050F8}{\Large\bullet}~\ce{N}$; $\color{#FF0D0D}{\Large\bullet}~\ce{O}$; $\color{#90E050}{\Large\bullet}~\ce{F}$; $\color{#FFFF30}{\Large\bullet}~\ce{S}$; $\color{#1FF01F}{\Large\bullet}~\ce{Cl}$; $\color{#248F8F}{\Large\bullet}~\ce{Ru}$; $\color{#9E63B5}{\Large\bullet}~\ce{Sb}$.

Reactions for the synthetic procedures adapted from the corresponding articles [2, 3]:

$$ \begin{align} \ce{SbF5(s) + SO2(l) &->[\pu{-40 °C}] SbF5 · SO2(sln)} \tag{R1} \\ \ce{[Ru^{II}(NH3)4(HSO3)2](aq) + 2 HCl(aq) &->[reflux][15 min] [Ru^{II}(NH3)4(SO2)Cl]Cl(aq) + 2 H2O(l) + SO2(aq)} \tag{R2} \end{align} $$

References

  1. Atkins, P. W. Shriver & Atkins’ Inorganic Chemistry; W.H. Freeman and Co.: New York, 2010. ISBN 978-1-4292-1820-7.
  2. Minkwitz, R.; Molsbeck, W.; Preut, H. Kristallstruktur von $\ce{SbF5 · SO2}$ / Crystal Structure of $\ce{SbF5 · SO2}.$ Zeitschrift für Naturforschung B 1989, 44 (12), 1581–1583. DOI: 10.1515/znb-1989-1218. (Free Access)
  3. Vogt, L. H.; Katz, J. L.; Wiberley, S. E. The Crystal and Molecular Structure of Ruthenium-Sulfur Dioxide Coordination Compounds. I. Chlorotetraammine(Sulfur Dioxide)Ruthenium(II) Chloride. Inorg. Chem. 1965, 4 (8), 1157–1163. DOI: 10.1021/ic50030a015.
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