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A molecule MX3 has three pi bonds of the p(pi)-d(pi) type and a pyramidal geometry. X belongs to the p-block and no coordinate bonds are present. Select the correct statement about this molecule.

1. The molecule is hypovalent
2. The central atom M has two lone pairs
3. X uses its d-orbitals for pi bond formation
4. The hybridisation of M is sp3

Correct answer: The hybridisation of M is sp3

Solution

Pyramidal MX3 means M has 3 bonding pairs and 1 lone pair => sp3 hybridised (tetrahedral electron geometry). In p(pi)-d(pi) bonding: X contributes its filled p orbital and M accepts using its empty d orbital (back bonding). So M uses its d-orbitals, not X. This rules out option C. No hypervalency (not more than 4 electron pairs in total) — option A (hypovalent, meaning fewer bonds) is wrong. Two lone pairs on M would give a different geometry (T-shaped for AX3E2) — option B is wrong. Option D (sp3 for M) is correct.

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