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With regard to a trigonal bipyramidal molecule, such as $\ce{PIBrClF_2}$, can there be any constituional isomers? I'm thinking no, because constitutional isomers differ in the connectivity of their atoms. The molecule only has single bonds. All are bound to a single central atom. There is no way for the connectivity of the atoms to change versus something such as ethanol, $\ce{C_2H_6O}$. I can think of two constitutional isomers off the bat:

$\ce{H_3CCH_2OH}$ and $\ce{H_3COCH_3}$ in which the connectivity of the atoms differ; in the former, we have a hydroxyl group; in the latter, we are missing the hydroxyl group.

Therefore, $\ce{PIBrClF_2}$ should not have any constitutional isomers but does have stereoisomers, correct?

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Correct, no constitutional isomers, but there are stereoisomers (different substituents in the apical and equatorial positions). Just for the record, what is your fifth substituent?

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  • $\begingroup$ Oops, good catch! Also I've always heard the apical position referred to as the axial position but both terms should work; they mean the same thing. $\endgroup$ – Dissenter May 18 '14 at 18:04
  • $\begingroup$ Yes, they mean the same thing. I guess it's another one of those "style" things (like 1-methylethyl vs. isopropyl). Chemists I hung around with would tend to use "apical" more for non-carbon systems, like your phosphorous example. $\endgroup$ – ron May 18 '14 at 18:09
  • $\begingroup$ True; I've only seen textbooks describe carbon systems as axial or equatorial. Never really seen them use the terminology for inorganic compounds but my professor seems to have appropriated the terminology to apply to inorganic compounds as well. $\endgroup$ – Dissenter May 18 '14 at 18:13
  • $\begingroup$ Apical did remind me of biology and "apical meristems." $\endgroup$ – Dissenter May 18 '14 at 18:13

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