Question

Identify the species, which does not exist

• A. \([SiF_6]^{2-}\)
• B. \([SiCl_6]^{2-}\)
• C. \([GeCl_6]^{2-}\)
• D. \([Sn(OH)_6]^{2-}\)

Hint:

Small central atoms like silicon can form stable hexacoordinate complexes with small ligands such as \(F^-\), but not with larger ligands such as \(Cl^-\).

Answer 1

The Correct Option is B

Step 1: Understand when hexacoordinate complexes of Group 14 elements form.
Group 14 elements (Si, Ge, Sn) can form 6-coordinate complexes using d-orbitals. However, the central atom must be large enough to accommodate 6 ligands without steric strain.
Step 2: Evaluate $[SiF_6]^{2-}$.
$F^-$ is the smallest halide (ionic radius ~133 pm). Six $F^-$ ions pack comfortably around Si in an octahedral arrangement. $[SiF_6]^{2-}$ is a well-known, stable species. It EXISTS.
Step 3: Evaluate $[SiCl_6]^{2-}$.
$Cl^-$ has a much larger ionic radius (~181 pm). Six large $Cl^-$ ions cannot pack around small Si without severe steric repulsion. Therefore, $[SiCl_6]^{2-}$ does NOT exist.
Step 4: Evaluate $[GeCl_6]^{2-}$ and $[Sn(OH)_6]^{2-}$.
Ge is larger than Si: $[GeCl_6]^{2-}$ EXISTS. Sn is much larger: $[Sn(OH)_6]^{2-}$ also EXISTS.
Step 5: Identify the non-existent species.
Only $[SiCl_6]^{2-}$ does not exist due to the size mismatch between small Si and large $Cl^-$.
Step 6: State the final answer.
\[ \boxed{[SiCl_6]^{2-}} \]