I need help understanding the following statement in my book (regarding trigonal bipyramidal geometry):

Placing a lone pair in an axial position would put it at 90 degree to three other electron domains. Placing it in an equatorial put is at 90 degree to only two other domains, thus minimizing the number of strong repulsive interactions

What does it mean by "90 degree" to two/three "domains"?


Here's a drawing of the situation.

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In the example with the axial lone pair of electrons, when they say, "90 degree to three other electron domains", they just mean that the axial lone pair is situated 90° relative to 3 other substituent bonds - the 3 equatorial substituent bonds in this case. When the lone pair is located in an equatorial position, then it has a 90° interaction with only 2 other bonds - the 2 axial substituent bonds in this case; the lone pair is 120° away from the 2 other equatorial substituents.


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