Mercury(II) forms very stable complexes with I⁻ and RS⁻ but weak complexes with F⁻ and OH⁻. How does HSAB theory explain this?
AHg²⁺ is a hard acid that prefers hard bases, but I⁻ and RS⁻ happen to have larger formation constants due to kinetic effects
BHg²⁺ is a soft acid (large, d¹⁰, highly polarizable) that forms strong bonds with soft bases (I⁻, RS⁻ — large, polarizable) rather than with hard bases (F⁻, OH⁻ — small, electronegative)
CHg²⁺ is small and highly charged, making it hard, but soft bases overwhelm it through their size
DHSAB does not apply to mercury; the trend is explained by simple electrostatic arguments
Hg²⁺ is the textbook soft acid: it has a large ionic radius, a filled d-shell (d¹⁰), low charge density, and high polarizability. Soft-soft combinations (Hg²⁺ with I⁻, RS⁻, CN⁻) are stabilized by covalent, orbital-overlap-driven interactions rather than electrostatic ones. Hard bases like F⁻ and OH⁻ are small and electronegative — they interact best with small, highly charged hard acids like Al³⁺ or Fe³⁺ through predominantly electrostatic bonds. This is why mercury poisoning targets sulfur-containing proteins (soft S donors) rather than oxygen-rich environments.
Question 2 True / False
Fe³⁺ is classified as a hard acid while Fe²⁺ is borderline. This difference arises because higher oxidation state increases charge density and decreases polarizability.
TTrue
FFalse
Answer: True
Hardness increases with charge density: higher charge on a smaller ion means stronger electrostatic interactions and less tendency for the electron cloud to distort. Fe³⁺ has higher charge and slightly smaller ionic radius than Fe²⁺, giving it greater charge density, less polarizability, and harder character. This is a general trend — for any metal, higher oxidation states produce harder acids. It explains why Fe³⁺ preferentially binds hard oxygen donors (as in rust, Fe₂O₃) while Fe²⁺ shows affinity for softer nitrogen and sulfur donors in biological systems.
Question 3 True / False
HSAB theory predicts that hard-soft mismatches always produce thermodynamically unstable complexes.
TTrue
FFalse
Answer: False
HSAB is a qualitative guideline, not an absolute rule. Hard-soft mismatches (like hard acid + soft base) tend to form less stable complexes than matched pairs, but 'less stable' does not mean 'unstable.' Many mismatched complexes exist and are perfectly stable — they are simply less stable than what matched combinations would produce. Additionally, factors like the chelate effect, steric constraints, and kinetic barriers can override HSAB predictions. The theory identifies preferences, not prohibitions.
Question 4 Short Answer
Using HSAB theory, explain why gold is found in nature as the native metal or in sulfide ores, while aluminum is found exclusively in oxide and hydroxide minerals (like bauxite), never as native metal or sulfide ores.
Think about your answer, then reveal below.
Model answer: Gold is a soft acid (Au⁺ and Au³⁺ have large radii, filled or nearly filled d-shells, high polarizability). Sulfur is a soft base (large, polarizable). The soft-soft match makes gold-sulfide compounds stable, explaining gold sulfide ores. Gold also has a very high reduction potential, preferring to remain as metallic Au⁰ rather than oxidize — hence native gold. Aluminum is a hard acid (Al³⁺ is small, highly charged, low polarizability). Oxygen is a hard base (small, high electronegativity). The hard-hard match produces extremely stable aluminum oxides and hydroxides (bauxite, corundum). A hypothetical aluminum sulfide would be a hard-soft mismatch and is thermodynamically unfavorable relative to the oxide. The correlation between mineral occurrence and HSAB matching is one of the theory's most striking successes in geochemistry.
This geological pattern extends broadly: the lithophile elements (those found in oxide/silicate ores) are predominantly hard acids, while the chalcophile elements (found in sulfide ores) are soft acids. HSAB theory elegantly rationalizes this entire geochemical classification.