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In one question for finding $\ce{P(CH3)2H3}$ structure? Which one should be placed in axial position? I think $\ce{-CH3}$ should be axial position to minimise repulsion.

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According to Inorganic chemistry (Meissler and Tarr), electronegativity and symmetry actually play a huge role. More electronegative groups tend to occupy axial positions, even if they are smaller, and symmetry is always preferred. For an example of the role of symmetry consider, $\ce{PF3(CF3)2}$ ($\ce{-CF3}$ are axial) vs $\ce{PF2(CF3)3}$ ($\ce{F}$ are axial). For an example of electronegativity being more important than size look at $\ce{PCl3F2}$ which has the $\ce{F}$ at the axial positions.

In this case, $\ce{-CH3}$ groups are both bigger, more electronegative, and putting them at axial positions will be symmetric (and cause the net dipole to cancel); with all these considerations, the $\ce{-CH3}$ groups will be axial.

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No, it's the other way around. Axial positions are less spacious, so you'd want to place your smaller substituents there and the bulky ones to the equatorial.

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  • $\begingroup$ But axial positions are longer than equitorial ones $\endgroup$ Aug 25 '15 at 19:56
  • $\begingroup$ Yes, and that's exactly because they are more "narrow", so the substituents occupying them are forced farther away from the center. $\endgroup$ Aug 25 '15 at 20:18
  • $\begingroup$ I'm afraid Pi5 is right you should cite some source if you think otherwise $\endgroup$
    – Mithoron
    Aug 25 '15 at 21:13
  • $\begingroup$ Would this count as a source? en.wikipedia.org/wiki/Trigonal_bipyramidal_molecular_geometry $\endgroup$ Aug 25 '15 at 21:15

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