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Is there any sort of AX3E2 molecule that consists of hydrogen atoms bonded to the main atom who is left over with two lone pairs of electrons? (Similar to ClF3 just with hydrogen instead of fluorine?)

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  • $\begingroup$ Probably doesn't exist. $\endgroup$
    – Mithoron
    Apr 28, 2018 at 18:59
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    $\begingroup$ I've not thought about it thoroughly but I also suspect it does not exist, for reasons similar to why SH6 doesn't exist. $\endgroup$ Apr 28, 2018 at 19:12

1 Answer 1

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We do have an example, with hydrogen bonded to ... ruthenium. The ternary hydride $\ce{Mg3RuH3}$ has been characterized via X-ray and neutron powder diffraction (with hydrogen in the form of deuterium) by Bonhomme et al. 1. The T shape is slightly distorted in the Y-like direction, but clearly closer to an ideal T than to a trigonally symmetric structure and having no evidence of pyramidalization. The shape is depicted in the figure taken from the reference.

enter image description here

The electronic structure is a nontrivial problem. The authors in Ref. 1 suggest an ionic formulation $\ce{[Mg^{2+}]3[RuH3^{6-}]}$, with the anion dimerized via a ruthenium-ruthenium bond to meet the 18-electron rule. However, an analysis of electron distribution by Firman and Landis 2 suggests that the magnesium is actually magnesium(I) ($\ce{Mg2^{2+}}$) and the ruthenium complex $\ce{RuH3^{3-}}$ consists of a covalently bound $\ce{RuH^-}$ center bonded predominantly ionically with the two flanking hydride ligands. If the $\ce{RuH^-}$ center is considered a unit isoelectronic with a silver ion (they both have $46$ electrons), then the complete complex would be valence-isoelectronic with a species such as $\ce{AgCl2^-}$.

References

  1. F. Bonhomme, K. Yvon, P. Fischer (1992). "Tetragonal trimagnesium ruthenium trideuteride, Mg3RuD3, containing dinuclear [Ru2D6]^{12-} complex anions". Journal of Alloys and Compounds, 186, 2, 309-314. ISSN 0925-8388, https://doi.org/10.1016/0925-8388(92)90017-4.

  2. Timothy K. Firman and Clark R. Landis 1998). "Structure and Electron Counting in Ternary Transition Metal Hydrides". J. Am. Chem. Soc. 120, 48, 12650–12656. https://doi.org/10.1021/ja982746r.

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