1. Consider the molecule dimethylsulfoxide (DMSO) with formula (CH₃)₂SO.

a) (5 points) Say as much as you can about the Lewis base properties of DMSO. Your answer should include at minimum a Lewis structure; indication of donor atoms; application of HSAB theory; stereochemistry at donor atoms in both base and a possible adduct.

• O, a hard donor, can bind to hard acids (preferred) or borderline acids; stereochemisty at O is indeterminate in DMSO, but will be bent in an adduct.
• S, a soft donor, can bind to soft acids (preferred) or borderline acids; stereochemistry at S is trigonal pyramidal in DMSO, tetrahedral in an adduct.

b) (5 points) Would you expect Pb²⁺ to form a stronger adduct with DMSO or with Cl₂SO, which has an analogous structure? Defend your choice. Pb²⁺ is a soft acid that will bind to the sulfur donor of either DMSO or Cl₂SO. Several arguments can be made (probably more than I have thought of!):

• Cl is more electronegative than C of CH₃ and will withdraw electron density from the sulfur atom, reducing its donor ability. (But remember: electronegativity is a fickle friend.)
• If we view DMSO as an adduct of SO²⁺ and either CH₃^- or Cl^-, we recognize that Cl^- is a harder donor than CH₃^-, and will harden the S atom in Cl₂SO relative to (CH₃)₂SO. This should lessen the potential affinity for the soft acid, Pb²⁺.
• However, Cl is capable of forming a pi bond with S, introducing resonance forms of the type below. This tends to enhance the electron density on S, making it a better donor. Whoops! What do we do now? Any other reasonable arguments?

2. (10 points) Indicate the predominant adducts present in an aqueous solution initially containing comparable concentrations of DMSO, Fe²⁺, Fe³⁺, NH₃, ClO₄^-, and K⁺. To repeat, list only the predominant adducts.