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GATE 2021 CY – Question 18

Inorganic Chemistry · Main Group Elements: Shapes and structures of molecules using VSEPR postulates, Hydrides, halides, oxides, oxoacids, nitrides, sulfides - shapes and reactivity. Structure and bonding of boranes, carboranes, silicones, silicates, boron nitride, borazine and phosphazenes: electron counting in polyhedral boranes, isolobal analogy. Allotropes of carbon, phosphorus and sulfur. Industrial synthesis of compounds of main group elements such as NH3, H2SO4 and HNO3. Chemistry of noble gases and interhalogen compounds. Acid-base concepts and principles (Lewis, Brønsted, HSAB and acid-base catalysis). · 1 mark · Multiple choice

$LiAlH_4$ reacts with one equivalent of $Me_3N\cdot HCl$ to give a tetrahedral compound, then another equivalent of $Me_3N\cdot HCl$ gives N. Identify N and its geometry.

  1. $LiAlH_4NMe_3$, trigonal bipyramidal
  2. $Li_2AlH_4Cl$, square pyramidal
  3. $AlH_3(NMe_3)_2$, trigonal bipyramidal
  4. $AlH_3(NMe_3)_2$, pentagonal

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Correct answer: (C) $AlH_3(NMe_3)_2$, trigonal bipyramidal

Explanation

Protonolysis of hydride by trimethylammonium chloride releases H2 and forms the neutral alane adduct AlH3(NMe3). Addition of a second donor amine gives five-coordinate **AlH3(NMe3)2**. Its five donor atoms adopt a trigonal-bipyramidal arrangement.

Therefore **C**, rather than a lithium-containing five-coordinate formulation.