${\left[\mathrm{S}\mathrm{i}\mathrm{C}{\mathrm{l}}_6\right]}^{-2}$ is unstable due to steric hinderance.

Steric hindrance is when the large size of groups within a molecule prevents chemical reactions which can take place in related molecules with smaller groups. Steric hindrance is also a reason for protection of some groups from unwanted chemical modifications.