Why does trisilylphosphine $\ce{P(SiH3)3}$ (ICSD Entry: 72676, [1]) have a pyramidal structure while trisilylamine $\ce{N(SiH3)3}$ (ICSD Entry: 201428, [2]) is planar?
I think since silicon has a vacant d-orbital, it would form a backbond, wherein phosphorus shares its lone pair, as is the case with trisilylamine. This would result in $\ce{sp^2}$ hybridization, i.e. a trigonal planar structure.
However, the crystal structure confirms the pyramidal $\ce{P}$ atom, which is also preserved in the gas phase [1]:
Although IR and Raman spectroscopic results (Davidson, Ebsworth, Sheldrick & Woodward, 1966) were consistent with trisilylphosphine being similar to trisilylamine in that the central atom was surrounded by a trigonal planar arrangement of $\ce{Si}$ atoms, electron diffraction studies (Beagley, Robiette & Sheldrick, 1968) established pyramidal geometry in the gas phase.
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The crystal structure of trisilylphosphine may be contrasted with those of trisilylamine and trimethylamine. As $\ce{N(SiH3)3}$ has a planar heavy-atom skeleton and concomitantly lacks a stereochemically active lone pair (Barrow & Ebsworth, 1984), the absence of discrete intermolecular $\ce{Si\bond{...}N}$ contacts — seen in solid dimethylsilylamine (Blake, Ebsworth & Welch, 1984a) and chlorosilyl-N,N-dimethylamine (Anderson, Blake, Cradock, Ebsworth, Rankin & Welch, 1986) — is unsurprising. Like $\ce{P(SiH3)3},$ trimethylamine is clearly pyramidal $[\angle \ce{CNC} = 110.40(7)^\circ]$ but the absence of vacant d orbitals on the $\ce{C}$ atoms removes the possibility of these acting as acceptors and the crystal comprises isolated molecules (Blake, Ebsworth & Welch, 1984b).
J.D. Lee Concise Inorganic Chemistry [3, p. 77] also specifically mentions that $\ce{P(SiH3)3}$ has a trigonal pyramidal structure:
In case of $\ce{P(SiH3)3},$ the internuclear distance is large and $\ce{P}$ atom has its own vacant d orbital. Hence the tendency to donate lone pair is very less and it adopts pyramidal structure and $\ce{P}$ atom is $\ce{sp^3}$ hybridized.
References
- Blake, A. J.; Ebsworth, E. A. V.; Henderson, S. G. D. Structure of Trisilylphosphine, $\ce{P(SiH3)3},$ at $\pu{100 K}.$ Acta Crystallogr C Cryst Struct Commun 1991, 47 (3), 486–489. DOI: 10.1107/S0108270190009593.
- Barrow, M. J.; Ebsworth, E. A. V. Crystal and Molecular Structure of Trisilylamine at $\pu{115 K}.$ J. Chem. Soc., Dalton Trans. 1984, No. 4, 563. DOI: 10.1039/dt9840000563.
- Sudarsan Guha. J.D. Lee Concise Inorganic Chemistry, for JEE (Main & Advanced), 4th ed.; Wiley India Pvt. Ltd.: India, 2017 (reprint 2020). ISBN 978-81-265-9114-5.
