Ab initio calculations with full geometry optimization are performed on a wide range of silicon atranes, RSi[-Y(CH2)(2)-]N-3, where R = H, F, OH, NH2, CH3, Cl, SH, PH2, SiH3, and Y = O, NH, NCH3, CH2, using the 6-31G(d) or larger basis sets. The results are used to show trends in the dative SiN bond as a function of axial R and equatorial Y substitution. The SiN bond is shown to be quite weak, making the atrane geometries very sensitive to medium effects, as shown by model solvation computations. The nature of the SiN bonding is best described as dative, as shown by localized orbitals.