To determine which molecule does not exhibit sp^3d^2 hybridization, we need to analyze each given molecule's hybridization based on its electronic configuration and molecular geometry.
Therefore, the molecule that does not exhibit sp^3d^2 hybridization is PF_5, which instead shows sp^3d hybridization.
Hybridisation and geometry of [Ni(CN)$_4$]$^{2-}$ are