3.1.6 [B 3 O 6 ]
3−
The only structure of the triborate ion [B 3 O 6 ]
3− , of D 3h symmetry, is given in Fig. 2.
This ion is characterized crystallographically in sodium, potassium, rubidium, and
cesium metaborate (see Tables 1, 2, 3, 4 and 5). There does not appear to be any
systematic trend in the range of the B-O distances upon varying the alkali metal.
3.2 Tetraborate Species
3.2.1 [B 4 O 5 (OH) 4 ]
2−
The structure of the tetraborate ion, [B 4 O 5 (OH) 4 ]
2−
, is given in Fig. 3. Initially, four
structures of C 2v symmetry were tried. None of these were energy minima. All of
these had both an A 2 and a B 2 imaginary mode, and, at some levels, some have a B 1
imaginary mode. The four structures of C 2 symmetry derived by desymmetrizing
along the A 2 mode were all minima, but only three unique structures were found, as
some coalesced. The four structures of C s symmetry derived by desymmetrizing
along the B 2 mode (#1–#4) coalesced into two different structures, but only C s #2
was a minima, at all levels except B3LYP/6-31G* and MP2/6-31G*. The four
structures of C s symmetry derived by desymmetrizing along the B 1 mode (#5–#8)
did not give minimum energy structures, and C s #8 ascended in symmetry to C 2v
C 2v
C 2
C s #1
C s #2
C 2v #1
D 3h
C 1 #1
C 1 #2
C 1 #3
C 2v #2
C 2
C s #1
C s #3
C 1 #1
Fig. 2 Structure of triborate ion, [B 3 O 3+n (OH) 6-2n ]
3− , n = 1 − 3
A Crystallographic Review of Alkali Borate Salts …
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