The search for silicon complexes that exhibit hydrolytic stability in neutral aqueous solution is a major challenge in contemporary silicon chemistry and has both technical and biochemical relevance. The recently published claim by one of us that a zwitterionic bis(diolato)silicate, derived from anhydroerythritol (meso-oxolane-3,4-diol; AnEryt) is hydrolytically stable at pH 7–8, thus appeared to be amilestone. The compound in question is bis[mesooxolane-3,4-diolato(2 )](morpholiniomethyl)silicate (1 (originally labeled 10 in Ref. [1]); see Figure 1). It was reported that no tetracoordinate silicon species could be detected upon dissolution of 1 in water; that is, no hydrolytic decomposition occurred. The hydrolytic stability that was claimed would have meant that 1 is drastically different from pentacoordinate bis[meso-oxolane-3,4-diolato(2 )]silicates, which do not have the morpholiniomethyl group as their fifth substituent. An example is the analogous pentacoordinate anionic species, bis[mesooxolane-3,4-diolato(2 )]hydroxosilicate. On the one hand, its alkali salts share with 1 a convenient synthetic accessibility from aqueous solution by simple evaporation of the solvent. On the other hand, these hydroxosilicates and the related phenylsilicates show partial hydrolysis in aqueous solution at high pH values, typically at about pH 12, and complete hydrolysis in neutral aqueous environment, as observed by NMR spectroscopy. Though it appears to be a reasonable hypothesis that the introduction of a positive charge close to the silicon center may suppress hydrolytic decomposition, the first step of which may be the protonation of a silicon-bonded alkoxide O atom, the claimed extent of stabilization of 1 seemed highly unlikely in light of a report by K5stele et al. on anionic pentacoordinate oxolanediolatosilicates. It thus appeared sensible to connect the areas of anionic and zwitterionic silicates in a step-by-step procedure in order to identify the origin of the apparent inconsistencies. The experimental procedure chosen is summarized in Figure 2. Starting from the well-known area of pentacoordinate oxolanediolatosilicate anions under non-hydrolyzing conditions, the experiments eventually approach the conditions reported in Ref. [1]. The NMR spectra of the anionic morpholinomethyl homologue of a bis(oxolanediolato)phenylsilicate or -hydroxosilicate in methanolic solution are shown in Figure 2a. Under these basic, non-hydrolyzing conditions, the conjugate base of 1, the anion [(AnErytH 2)2SiCH2NC4H8O] , was expected to be the only reaction product. In fact, the signal patterns in both the Si and C NMR spectra (Figure 2a) consistently resembled the spectra of the analogous phenylor hydroxosilicates. The spectra in Figure 2a thus show all the typical features related to the characteristic syn/anti isomerism. First, the Si NMR spectrum consists of three signals that span a typical chemical shift range of some 3–8 ppm. The signal of the minor component, tentatively the syn/syn isomer, appeared, as expected, most upfield. The syn/anti and anti/ anti isomers were the main species. Crystals of K[(AnErytH 2)2SiCH2NC4H8O]·MeOH grown from such solutions accordingly contained one of the two main isomers, namely the anti/anti-configured anion. Second, the C NMR spectrum (Figure 2a) revealed a spectral feature corresponding to the phenylsilicate analogue: The C NMR signals of the monovalent substituent at the silicon center mirrored the pattern of the Si NMR signals. The signals assigned to C6 and C7 showed the syn/syn minor component clearly separated and shifted upfield by about 1 ppm from a pair of signals corresponding to the main isomers. The signal for C5, corresponding to one carbon atom only, appears to be a less suitable probe owing to its lower intensity, a situation that parallels the difference in usefulness of the Cipso and Cortho signals of phenylsilicates. [3] Figure 1. The zwitterionic (oxolanediolato)silicate 1. The orientation of an oxolane ring with respect to the monovalent substituent at the silicon center defines syn or anti bonding. The numbering of the oxolane carbon atoms corresponds to that for a furanose.