What is the formula of the complex lithium tetrahydridoaluminate(III)?

The correct answer is Option (2) → Li[AlH₄]

The formula is built back from the name: the cation is named first, the ligands of the complex ion next, and the ending -ate marks a complex anion.

• Cation: lithium, \( \ce{Li^+} \).

• Ligands: tetrahydrido = four hydride ions, \( \ce{H^-} \) (an anionic ligand, so the name ends in -ido).

• Central atom: aluminate = aluminium in a complex anion, with oxidation state +3.

• Charge of the complex ion: +3 + 4(−1) = −1, so it is \( \ce{[AlH4]^-} \).

• Charge balance: one \( \ce{Li^+} \) balances one \( \ce{[AlH4]^-} \), giving \( \ce{Li[AlH4]} \).

Why the other options are incorrect

Option (1) Li₃[AlH₆] — Incorrect. Six hydride ligands would be hexahydrido, not tetrahydrido.

Option (3) Al[LiH₄] — Incorrect. “Aluminate” makes aluminium the central atom of the complex anion and lithium the cation; this formula swaps them.

Option (4) Li₂[AlH₄] — Incorrect. \( \ce{[AlH4]^-} \) carries only −1, so two \( \ce{Li^+} \) leave a net +1: the charges do not balance.

NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “… If the complex ion is an anion, the name of the metal ends with the suffix – ate.”

“Note: The 2004 IUPAC draft recommends that anionic ligands will end with –ido so that chloro would become chlorido, etc.”

Example 5.2 – “Write the formulas for the following coordination compounds: … (b) potassium tetrahydroxidozincate(II) …
(b) K₂[Zn(OH)₄]”