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I can understand why R₃N=O is 'rare' since for this double bond to form, Nitrogen would have to recruit a d-orbital like phosphorous does (the charge-separated model) but for N the d-orbitals aren't as accessible. That being said, I don't understand why R₂Si=O is rare since surely d-orbitals are just as accessible for Si as they are for P.

Would appreciate some clarification :)

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    $\begingroup$ Another question boiling down to putting "d-orbitals" and their double bonds where the sun doesn't shine :/ $\endgroup$
    – Mithoron
    Apr 11 at 16:49

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$\ce{R₂Si=O}$ are referred to as silanone and are extremely reactive. There is an explanation of this in the Wikipedia article:

Silanones are extremely reactive and until 2013 were only detected by argon matrix isolation or in the gas phase but not isolated. A synthesis of a stable silanone was reported in 2014**. Silanones are of some interest to academic research, with their reactivity being of some relevance to the double bond rule.

Silanones are unstable and favor oligomerisation to siloxanes. The reason for this instability is the weak pi bond with a small HOMO–LUMO energy gap caused by an unfavorable overlap between the p-orbitals of silicon and oxygen. A second reason for the observed instability is the strongly polarized silicon–oxygen bond, $\ce{Si^{δ+}–O^{δ−}}$.

**In the last decade, there were reports of stable silanones getting synthesized involving bulky groups. Check below papers for more details:

  1. Sen, S.S. (2014), A Stable Silanone with a Three-Coordinate Silicon Atom: A Century-Long Wait is Over†. Angew. Chem. Int. Ed., 53: 8820-8822. DOI: 10.1002/anie.201404793
  2. Silicon and Oxygen’s Bond of Affection: An Acyclic Three-Coordinate Silanone and Its Transformation to an Iminosiloxysilylene Daniel Wendel, Dominik Reiter, Amelie Porzelt, Philipp J. Altmann, Shigeyoshi Inoue, and Bernhard Rieger Journal of the American Chemical Society 2017 139 (47), 17193-17198 DOI: 10.1021/jacs.7b10634
  3. Filippou AC, Baars B, Chernov O, Lebedev YN, Schnakenburg G. Silicon-oxygen double bonds: a stable silanone with a trigonal-planar coordinated silicon center. Angew Chem Int Ed Engl. 2014 Jan 7;53(2):565-70. doi: 10.1002/anie.201308433. Epub 2013 Nov 29. PMID: 24288183.
  4. Synthesis and structure of a pyridine-stabilized silanone molybdenum complex and its reactions with PMe3 and acetone, Takako Muraoka, Yuzuki Suzuki, Masato Tsuchimoto, Gama Trigagema, Keiji Uenoa and Shinji Koyama, Dalton Trans., 2022 ,51, 18203-18212, DOI:10.1039/D2DT02560D
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