1
$\begingroup$

How can I determine if a coordination compound has a tetrahedral or square planar geometry without any experimental data (such as magnetic spin) by just looking at its formula?

For example, what is the geometry of the following complexes:

  • $\ce{[CoCl4]^2-}$
  • $\ce{[Ni(CN)4]^2-}$
  • $\ce{[CuCl4]^2-}$

Do they have a Jahn-Teller distortion?

$\endgroup$
  • 6
    $\begingroup$ Your question is like asking "how do I tell whether an object weighs more than a kilogram without measuring its mass". For some objects, it's obvious: a feather is definitely under 1 kg, an elephant definitely over. For some other things, your eyes aren't good enough and the only way to know is to put it on a scale. A more nuanced question, and probably better, would be to ask what factors tend to favour tetrahedral geometries vs square planar geometries. Some of them are touched upon here: chemistry.stackexchange.com/q/40880/16683 $\endgroup$ – orthocresol Jun 27 '19 at 13:51
  • $\begingroup$ Thank you for your time. I know that question is a bit odd but this is a question from my last inorganic chemistry exam. Well, let me write that question exactly, Wich one of those ligands you can observe Jahn-Teller Distortion? and why? [CoCl4]2− [Ni(CN)4]2− [CuCl4]2− $\endgroup$ – İsmail Burak AYDIN Jun 27 '19 at 15:59
  • 1
    $\begingroup$ As speculation: if it was an examination question, perhaps especially an oral one, see it as a check about «how would you answer this later in professional life by yourself»: display both the pathway (your reasoning «because of chlorine ligands ...») as well as your answer («so x and y were square panar, z however tetrahedral») plus a probe («which could be experimentally confirmed by method A»). For your comment, put forward your new arguments -- even if they do not lead yet to an answer, it will 1) show us where to help you and 2) show you to yourself what you already understood. $\endgroup$ – Buttonwood Jun 28 '19 at 21:34
  • $\begingroup$ For example [Ni(CN)4]2− I've drawn energy diagram for that compound both tetrahedral and square planar, put electrons with respect to strong field ligand and as you did say and I've made consumptions about Jahn-Teller distortion for both geometries. What i couldn't get is, if this the right way to do it? Note that i don't have any extra data $\endgroup$ – İsmail Burak AYDIN Jun 29 '19 at 13:29