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 .
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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 .
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