I have seen $\ce{CuCl}$ and $\ce{Cu2Cl2}$ used interchangeably; they seem to be equivalent. If that is the case, then why does the latter have the '$2$' subscripts?

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    $\begingroup$ Without more context, this is hard to judge. Consider $\ce{Hg2Cl2}$, which has a polycation. $\endgroup$ – TAR86 Feb 1 '17 at 11:47
  • $\begingroup$ I was looking at Sandmeyer reaction catalysts (high school course). $\endgroup$ – dark32 Feb 1 '17 at 11:50
  • $\begingroup$ For this very special case, it might be that the typesetting is off (This is a shot in the dark). In the Sandmeyer reaction, Cu changes its oxidation state between +1 and +2. Could the subscripts erroneously refer to this? Please reevaluate the material with this in mind. $\endgroup$ – TAR86 Feb 1 '17 at 15:22

$\ce{Cu2Cl2}$ is actually the dimer of cuprous(I) chloride $\ce{CuCl}$. Cuprous chloride dimerises and also trimerises and tetramerises in gas phase. The molecular structures of the species are measured with Gas-Phase Electron Diffraction and Quantum Chemical Calculations. Some information about the various species of cuprous chloride is given here:[1]

The molecular geometry of gaseous cuprous chloride oligomers was determined by gas-phase electron diffraction at two different temperatures. Quantum chemical calculations were also performed for $\ce{Cu_nCl_n (n=1–4)}$ molecules. A complex vapor composition was found in both experiments. Molecules of $\ce{Cu3Cl3}$ and $\ce{Cu4Cl4}$ were present at the lower temperature (689 K), while dimeric molecules ($\ce{Cu2Cl2}$) were found in addition to the trimers and tetramers at the higher temperature (1333 K). All $\ce{Cu_nCl_n}$ species were found to have planar rings by both experiment and computation. The bond lengths from electron diffraction ($r_\mathrm g$) at 689 K are 2.166±0.008 Å and 2.141±0.008 Å and the $\ce{Cu-Cl-Cu}$ bond angles are 73.9±0.6° and 88.0±0.6° for the trimer and the tetramer, respectively. At 1333 K the bond lengths are 2.254±0.011 Å, 2.180±0.011 Å, and 2.155±0.011 Å, and the $\ce{Cu-Cl-Cu}$ bond angles 67.3±1.1°, 74.4±1.1°, and 83.6±1.1° for the dimer, trimer, and tetramer, respectively.


(1) Hargittai, M.; Schwerdtfeger, P.; Réffy, B.; Brown, R. The Molecular Structure of Different Species of Cuprous Chloride from Gas-Phase Electron Diffraction and Quantum Chemical Calculations. Chem. Eur. J. 2003, 9 (1), 327–333. DOI: 10.1002/chem.200390027

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  • $\begingroup$ So, CuCl has no real existence? And is it a covalent molecule? $\endgroup$ – user226375 May 23 '19 at 4:34
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    $\begingroup$ @user226375 it is a solid at room temperature. It just polymerizes in gas phase. It is ionic in nature and has various crystalline forms. $\endgroup$ – Nilay Ghosh May 23 '19 at 8:27
  • $\begingroup$ thanks for replying. Is it ionic in even gas phase?( where it exists in dimer form) $\endgroup$ – user226375 May 23 '19 at 11:03
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    $\begingroup$ @user226375 no it is not ionic in gas phase and it polymerizes $\endgroup$ – Nilay Ghosh May 23 '19 at 16:01

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