This is regarding a situation where an atom has five electron domains; two domains are lone pairs, and three are bonds to other atoms.

The course materials I am using (focusing on VSEPR theory) suggest that, as a rule of thumb, T-shaped geometry is the configuration the electron domains around an atom will take in the above situation. However, that feels unintuitive to me, because the separation between the lone pairs (which ought to repel everything else more strongly) is not maximised in that configuration - it makes more sense in my head that a trigonal planar geometry of bonds would be adopted, with a lone pair above and below the plane of the bonds, to minimize the repulsion between those two lone pairs, as well as to distance the three bonded atoms further apart.

Why, then, is T-shaped geometry preferred in most cases? Is that really the case, or a misrepresentation on the part of the course materials? Many thanks.

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    $\begingroup$ In each scenario, for each lone pair, count the neighbors at 90°. $\endgroup$ – Zhe Jan 20 at 4:56
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    $\begingroup$ You can look again at the schematic and see that the trigonal bipyramidal minimise the global repulsion as no 90° angles are between the T arms and the lone pairs. But it is true that it is the more difficult case to visualise quickly. $\endgroup$ – Alchimista Jan 20 at 10:49

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