Semiconductor nanocrystals (NCs) serve as useful building block materials made attractive by their solution processibility and size dependent optical properties. In this thesis, I begin by introducing the fundamental properties and applications of semiconductor NCs and the use of X-ray scatting to probe NC solution properties. Chapter 2 focuses on the synthesis of high quality PbS NCs and their ligand exchange into polar solvents with multivalent metal chalcogenide complex (MCC) ligands. I explore the chemical analysis of the NC surface chemistry and how it impacts the colloidal stability of NCs in N-methylformamide solvents. Small angle X-ray scattering analysis is used to probe the concentration dependent solution structure factors and quantify the repulsive interaction potentials which are largest for intermediate (~6 nm) PbS NCs. In the second half of the thesis, Chapter 3 focuses on the transition of these electrostatically stabilized NCs from the solution to solid phase. Long-range order emerges as NCs flocculate and self-assemble into densely packed 3D supercrystals with interparticle spacings less than 0.5 nm. Supercrystals predominantly nucleate into face centered cubic (fcc) structures and can be dissolved in fresh solvent to recover the initial colloidal NCs without any change to the NC size or shape. I find that the supercrystal nucleation proceeds through both one-step and two-step mechanisms where supercrystals nucleate from a dense fluid phase in the latter case. The position of the NC solution within a phase diagram and the proximity to the binodal line separating dilute and dense fluid phases dictate the crystalline yield and kinetics of supercrystal nucleation. I also find that faceted PbS NCs assemble with atomic coherence between their atomic crystal structures across micron sized supercrystals, a feature dictated by the high surface charge density on the faceted NC surfaces.