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In general, the anions have bigger ionic radii than their corresponding neutral atoms, and the cations have smaller ionic radii then their corresponding neutral atoms. How come that does not apply to $\ce{C2-}$ and $\ce{C2+}$?

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  • $\begingroup$ What other anion/cation are you talking about? And are they mono-atomic or diatomic? $\endgroup$ – SteffX May 29 '17 at 15:23
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    $\begingroup$ The distance between two C atoms has nothing to do with the ionic radii in the first place. It is a covalent bond. $\endgroup$ – Ivan Neretin May 29 '17 at 15:23
  • $\begingroup$ @IvanNeretin how do I then determine the distance between the C-atoms? or at least which on is bigger? $\endgroup$ – DUDEofDK May 29 '17 at 17:38
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    $\begingroup$ You consider the molecular orbitals and deduce the bond order. The higher it is, the shorter is the bond. $\endgroup$ – Ivan Neretin May 29 '17 at 18:51
  • $\begingroup$ I assume you are concerned with the diatomic molecules/ions; could you provide some data for your claim? $\endgroup$ – Martin - マーチン Jun 1 '17 at 11:28
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Looking up in a current compilation of bond lengths in the International Tables for Crystallography (IT), apparently there is indeed not so much deviation in the distance between two adjacent carbon atoms in a $\ce{C(sp)#C(sp)}$ pattern:

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(IT, 2006 edition)

Some hints to render this excerpt easier to access. The IT indeed discern different bond types (13 for all combinations of $\ce{C-C}$, and 625 for all organic / organometallic compounds), and consequently, the source offers separate listings for, for example, $\ce{C(sp^3)-C(sp^2)}$ or $\ce{C(sp^2)-C_{ar}}$, too (first column). The second column is about the structure around the pattern in question highlighted in bold. For the $IT$, $\ce{C_{ar}}$ explicitly represents an aryl C in an six-membered ring; $\ce{C^{\#}}$ any $\ce{C(sp^3)}$ - linearly substituted, or not.

The third, fourth and fifth column are the unweighted sample mean $d$ in [Å], sample median $m$, and sample standard deviation $\sigma$, followed by the lower and upper quartile, $q_l$ and $q_u$, as well as the number of observations $n$. Overall, the compilation is based on a subset of 10 324 X-ray and neutron diffraction data found in the Cambridge Crystal Database by September 1985, judged to be "good enough" (the source offers additional insight about the criteria applied).

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