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My chemistry teacher said hydrogen when bonded with metals has oxidation number (O.N.) of −1 except CuH: in CuH the hydrogen has O.N. +1 and the copper has O.N. −1.

But on the internet it is ubiquitously told that the O.N. of hydrogen is −1 in CuH too, and the compound's name is copper(I) hydride. I am confused which one is correct. If hydrogen has O.N. of +1 in CuH, why is it so?

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    $\begingroup$ chemistry.stackexchange.com/questions/118739/… $\endgroup$
    – Mithoron
    Jun 18 at 21:52
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    $\begingroup$ Point to note, copper hydride is non stoichiometric i.e. it is $\ce{CuH_n}$ where n = 0.95 $\endgroup$ Jun 19 at 2:45
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    $\begingroup$ This turned out to be a surprisingly tough cookie to give a bulletproof answer to. Thoughts based on "hey, they called it copper(I) hydride here" and products of the reaction with HCl obviously cannot be considered as real answers. We appear to lack data for appropriate methods for oxidation number determination such as XPS or BVS based on crystal structure (bond valence parameters are scarce for copper(I)). Your teacher probably were thinking about something else while providing you with the answer and likely had an image of the chart for reactivity of the metals hanging in front of them. $\endgroup$
    – andselisk
    Jun 19 at 19:52
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    $\begingroup$ @andselisk I read your comment and just decided to "self-discipline" my answer. But I will note that the teacher likely did confuse the EMF series with electronegativity. In reality, hydrogen is higher in the series than its electronegativity "should" allow because of the great solvation energy of aqueous hydrogen ion. $\endgroup$ Jun 19 at 19:59
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    $\begingroup$ @OscarLanzi Yes, that was my thought too. And after thinking and researching for a while, I came to the conclusion that the question is actually tricky and it could be interesting to get a real solid answer for. $\endgroup$
    – andselisk
    Jun 19 at 20:02

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Korzhavi and Johanssen 1 performed first-principles calculations on the electronic energy levels of several compounds of copper, hydrogen and oxygen. They find good agreement between derived and experimental properties of stable compounds such as $\ce{Cu2O}$ and correctly predict the instability of other candidate compounds such as $\ce{CuOH}$ and $\ce{CuH2}$.

For $\ce{CuH}$ the calculated density of electron states shows the hydrogen-based 1s states filled, and lower in energy than the copper-based 3d states. This is similar to the oxygen- and hydroxyl-based states in the oxide and hydroxide. The copper hydride result thereby identifies that compound as a metal hydride with positive metal oxidation state.

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Reference

  1. Korzhavyi, Pavel & Johansson, Borje. "Thermodynamic properties of copper compounds with oxygen and hydrogen from first principles." Report to Svensk Kärnbränslehantering AB, Texhnical Report TR-10-30, February 2010. Retrieved 19 June 2022.
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