Chemistry - Mismatch of plane of symmetry vs Gold Book's definition for a pseudochiral carbon
The Gold Book definition for a meso compound is
the achiral member(s) of a set of diastereoisomers which also includes one or more chiral members.
So, a meso compound has chiral subsets. Here we would have three such subsets:
- Leftmost carbon (2R)
- The centermost (pseudochiral 3S)
- The rightmost (4S) carbon
Even though the centermost carbon is pseudo chiral, the plane of symmetry still exists. The compound remains meso, achiral.
Nomenclature is just for naming a compound.
The following two molecules are diastereomers, not enantiomers.
For the examples I've looked at, a small (r) or (s) is used for the pseudochiral carbon instead of (R) or (S). For example, see this section in Wikipedia. That means that this is not a real stereocenter. But we do need to assign something because there is intrinsic stereochemistry because it has relative stereochemistry with the two enantiomeric substituents, and changing the center produces diastereomers. As you noted, the reason it's a pseudocenter is because it cannot be a full stereogenic center by symmetry.
(2R,3s,4S)-Pentan-2,3,4-triol is a meso compound owing to the plane of symmetry. C2 and C4 are both chirotopic and stereogenic. Thus the R/S designation. C3 is ACHIROTOPIC and stereogenic (pseudoasymmetric). Hence, the lower case "s" (C3-OH>C2-R>C4-S>H, which are the CIP priorities). Inversion of C3 leads to a new meso diastereomer (2R,3r,4S) that has the same chirotopicity and stereogenicity at the three centers as did the original triol. To invoke enantiomers one needs the pair (2R,4R) and (2S,4S). C3 in each of these enantiomers has no designation (non-stereogenic and chirotopic) because inversion returns the same enantiomer. For a discussion of stereogenicity and chirotopicity in lieu of "pseudoasymmetry", see Mislow, K. and Siegel, J., J. Am. Chem. Soc., 1984, 106, 3319.