To determine which species is not stable, we need to consider the stability of the given complexes based on the central atom and its ability to accommodate ligands.$\\$Let's analyze each option:$\\$• Option 1: $[SiF_6]^{2-} \\$Silicon (Si) can form stable hexafluoro complexes due to the small size and high electronegativity of fluorine, which allows for strong Si-F bonds.$\\$• Option 2: $[GeCl_6]^{2-} \\$Germanium (Ge) can form stable hexachloro complexes. Although Ge is larger than Si, it can still accommodate six chlorine atoms due to its ability to expand its coordination number.$\\$• Option 3: $[Sn(OH)_6]^{2-} \\$Tin (Sn) can form stable hexahydroxo complexes. Sn is capable of expanding its coordination number to accommodate six hydroxide ions.$\\$• Option 4: $[SiCl_6]^{2-} \\$Silicon (Si) typically does not form stable hexachloro complexes. The Si-Cl bond is weaker compared to Si-F, and the larger size of chlorine compared to fluorine makes it difficult for Si to accommodate six Cl atoms.$\\$Therefore, the species that is not stable is $[SiCl_6]^{2-}. \\$This corresponds to Option 4.