Jc. Martin et al., HYPERVALENT PHOSPHORUS SPECIES AS AROMATIC AND METALLIC SPECIES, Phosphorus, sulfur and silicon and the related elements, 76(1-4), 1993, pp. 347-350
Hypervalent species are much better with equatorial groups which help
the plus charge on the central atom. The apical atoms have negative ch
arge and require groups to make the negative species stable. This is s
hown.