Tetrametaphosphimate conformers

The dihydrate of the tetramer is particularly stable and is, in fact, the bishydroxonium salt of tetrametaphosphimic acid [H30]J[(NH)4P4-06(0H)2] the anion of which has a boat configuration and is linked by short H bonds (246 pm) into a two-dimensional sheet (Fig. 12.29). The related salts M4[NHP02)4].- H20 show considerable variation in conformation of the tetrametaphosphimate anion, as do the 8-membered heterocyclic tetraphosphazenes (NPX2)4 (p. 537), e.g. 

The crystal structure determinations of tetrametaphosphimates showed that N-H 0 hydrogen bonds between the ring anions play an important structure directing role. The iPCfNH) rings adopt approximately a chair, boat, or saddle conformation. Examples for conformations in (P02NH)t ions as well as some structural aspects of tetrametaphosphimates are given in Fig. 3. 

Fig. 3. Examples for conformations in (P02NH)f ions (1, saddle 2, chair 3, distorted boat 4, crown) as well as some structural aspects (formation of strings (5) and of columns (6) by N-H O hydrogen bonding) of tetrametaphosphimates.

IR and Raman investigations by Steger et al. established the possible ring conformations of tri- and tetrametaphosphimate ions (74, 80, 101-103). Based on these results, interpretations of vibrational spectra (104, 105) as well as calculations of force constants in group III, transition metal, and rare earth tri- and tetrametaphosphimates were performed by Sukova et al. by least squares methods (106-110). Furthermore, calculations by the CNDO/2, MNDO, and PM3 methods were made (111-114). In addition, EPR spectra of some trimetaphos-phimates were recorded (115). P NMR experiments were used to investigate the formation and decomposition of polymetaphosphimates (60, 116-119), as well as for the protonation behavior of mono-, di-, and tri-ju.-imido cyclotriphosphates (120). The protonation was also studied potentiometrically (121). Al NMR experiments (122, 123)... 

The tetrametaphosphimate, (OaPNHlg, ring appears in the potassium tetrahydrate and caesium hexahydrate salts. The ring conformation is different in the two cases, having the chair form with almost 1/m symmetry in the former and the saddle form with 42w symmetry in the latter. Bond lengths and angles in the two conformers are very similar, with P-N at 1.673... 

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