Chlorine can be $sp^3d^3$ hybridized.
If so, it can form $\ce{ClH_7}$ and then chlorine, being more electronegative, will have (-7) as its oxidation number. But we know that the oxidation number of $\ce{Cl}$ lies between (-1) and (+7).
Thoughts?
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http://chemistry.stackexchange.com/questions/444/can-an-atom-have-more-than-8-valence-electrons-if-not-why-is-8-the-limit – Mithoron Jun 10 '15 at 21:40
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http://chemistry.stackexchange.com/questions/31584/on-the-geometry-of-sp3d-and-sp3d3-hybridization – Mithoron Jun 11 '15 at 12:55
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I would not be so quick to dismiss the existence of other hydrogen chlorides. – Oscar Lanzi Jul 26 '20 at 19:12
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Your question is based on a skewed premise, as gsurfer04 already pointed out. Rather than invoking d-orbitals, which are usually rather far away in energy from the corresponding p-orbitals (in fact, usually higher in energy than the next shell’s s-orbital, so for chlorine 3p → 4s → 3d), these hypervalent halogen compounds are considered to derive from multi-centre bonds.
For details in the bonding mechanism, I am going to forward you to this excellent answer.
I haven’t seen any multi-hydrogen chloride yet. In fact, when discussing interhalogens like $\ce{ClF3}$, our professor in the introductory chemistry course went as far as saying even $\ce{H3Cl}$ is impossible. I’ve forgotten the exact reasoning (and unfortunately it’s not explicitly stated in the web material any more) but I believe it to be along the lines of hydrogen’s s-orbitals not being able to form the outside parts of a four-electron three-centre bond.
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First of all, $d$ orbitals play no part in hypervalency.
If you look at existing hypervalent molecules, you'll see that the central atom is always less electronegative. As H is less electronegative than $\ce{Cl}$, $\ce{ClH7}$ will be very unstable.
gsurfer04
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7The central atom isn't always less electron negative http://pubs.acs.org/doi/abs/10.1021/ja00293a020 and http://pubs.acs.org/doi/pdf/10.1021/ja961081v – DavePhD Jun 10 '15 at 15:04
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@DavePhD Fascinating find on the triprotonated methane! Thanks for the link. – Nicolau Saker Neto Jun 10 '15 at 17:07
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@DavePhD Although the central atom isn't necessarily less electronegative, in order for the molecule to have 3c-4e bonds, theoretically, the peripheral atoms should be electronegative atoms to withdraw electron density away from the central atom. After all, two of the electrons of the 4 electrons in the 3c-4e bond becomes a lone pair on the peripheral atoms. – Tan Yong Boon Nov 26 '17 at 03:13