characterized ortho- and metaboric acid, tetrahydroxoborate, trioxoborate, and
dioxomonohydroxoborate were calculated ab initio, in addition to the postulated
conjugate base of boric acid, oxodihydroxoborate [BO(OH) 2 ]
−
, as well as the
transition structure leading to it. The effect of additional water was also calculated.
In this paper, the focus is on the polyborates.
The crystal structures of numerous sodium borates are given in Table 1. The
notation (l:m:n) is shorthand for a substance of oxide formula lM 2 O:mB 2 O 3 :nH 2 O,
where M is an alkali metal. Anhydrous (3:1:0) consists of discrete BO 3
3− ions [2].
The hydrated sodium borate (2:1:1) consists of discrete [BO 2 (OH)]
2− ions connecting sheets of edge-shared NaO polyhedral [3]. There are several hydrates of
sodium metaborate (1:1:x). The octahydrate [4] consists of the tetrahedral [B
(OH) 4 ]
− anion, as does the tetrahydrate [5]. (3:3:4) consists of the [B 3 O 4 (OH) 4 ]
3−
anion with two tetrahedral and one trigonal boron atoms [6]. (3:3:2) consists of
isolated [B 3 O 5 (OH) 2 ]
3− ions with two trigonal and one tetrahedral ion [7]. The
anhydrous sodium metaborate (1:1:0) contains discrete B 3 O 6
3− anions in which all
boron atoms are trigonal [8]. (2:3:1) consists of the unique [B 12 O 20 (OH) 4 ]
8− anion,
which consists of 6 6-membered rings spirofused together in a cycle. There are 6
tetrahedral and 6 trigonal boron atoms. On the outer periphery of the ion, four of the
trigonal boron atoms have hydroxyl groups and two have oxo groups [9]. Both
(3:5:4) [10] and (3:5:2) [11] consists of layers of B 5 O 6 polyhedra containing three
tetrahedral and two trigonal boron atoms.
Borax (1:2:10) [12], also known as sodium tetraborate decahydrate, consist of
chains of edge-shared hexaaquasodium octahedra alternating with chains of discrete
[B 4 O 5 (OH) 4 ]
2− ions hydrogen-bonded to each other [13]. The tetraborate anion
consists of an oxygen-boron core with two tetrahedral and two trigonal boron atoms
reminiscent of bicyclo [3.3.1] nonane, where the bridgeheads correspond to the
tetrahedral boron atoms. The structure can therefore be reformulated as
Na 2 [B 4 O 5 (OH) 4 ] ⋅ 8H 2 O. The structure was refined by neutron diffraction [14].
The determination of the hydrogen atom positions indicate that the ion has C 2
symmetry [15]. The oxo atoms of the anion accept hydrogen bonds either from the
aquasodium chains (cross-chain) or from the hydroxyl groups (in-chain), and the
hydroxyl group donate cross-chain hydrogen-bonds to the water oxygen atoms.
Sodium tetraborate “pentahydrate” [16] (the mineral tincalconite, 1:2:4.667) is a
reversible dehydration product of the decahydrate. The [B 4 O 5 (OH) 4 ]
2− ions persist
in this structure, however, the hydroxyls of the tetrahedral boron atoms are now
directly coordinated to the sodium atoms. The structure can therefore be reformulated as Na 2 [B 4 O 5 (OH) 4 ] ⋅ 3H 2 O. The hydroxyls of the trigonal boron hydrogen
bond to the oxo group between the two tetrahedral boron atoms. Upon refinement,
incomplete occupancy of the water sites was found, corresponding to the formula
Na 2 [B 4 O 5 (OH) 4 ] ⋅ 2.667H 2 O [17]. The disorder was also confirmed at room
temperature [18]. It was suggested that sodium tetraborate tetrahydrate (the mineral
kernite, 1:2:4) was similar to borax in structure [19]; however this was shown to be
incorrect, as the boron units formed an infinite chain of six-membered rings
comprised of [B 4 O 6 (OH) 2 ] n
2n− [20, 21]. A later redetermination also included an
electron population analysis [22]. The monohydrate consists of chains of
108
C. C. Pye
dioxomonohydroxoborate were calculated ab initio, in addition to the postulated
conjugate base of boric acid, oxodihydroxoborate [BO(OH) 2 ]
−
, as well as the
transition structure leading to it. The effect of additional water was also calculated.
In this paper, the focus is on the polyborates.
The crystal structures of numerous sodium borates are given in Table 1. The
notation (l:m:n) is shorthand for a substance of oxide formula lM 2 O:mB 2 O 3 :nH 2 O,
where M is an alkali metal. Anhydrous (3:1:0) consists of discrete BO 3
3− ions [2].
The hydrated sodium borate (2:1:1) consists of discrete [BO 2 (OH)]
2− ions connecting sheets of edge-shared NaO polyhedral [3]. There are several hydrates of
sodium metaborate (1:1:x). The octahydrate [4] consists of the tetrahedral [B
(OH) 4 ]
− anion, as does the tetrahydrate [5]. (3:3:4) consists of the [B 3 O 4 (OH) 4 ]
3−
anion with two tetrahedral and one trigonal boron atoms [6]. (3:3:2) consists of
isolated [B 3 O 5 (OH) 2 ]
3− ions with two trigonal and one tetrahedral ion [7]. The
anhydrous sodium metaborate (1:1:0) contains discrete B 3 O 6
3− anions in which all
boron atoms are trigonal [8]. (2:3:1) consists of the unique [B 12 O 20 (OH) 4 ]
8− anion,
which consists of 6 6-membered rings spirofused together in a cycle. There are 6
tetrahedral and 6 trigonal boron atoms. On the outer periphery of the ion, four of the
trigonal boron atoms have hydroxyl groups and two have oxo groups [9]. Both
(3:5:4) [10] and (3:5:2) [11] consists of layers of B 5 O 6 polyhedra containing three
tetrahedral and two trigonal boron atoms.
Borax (1:2:10) [12], also known as sodium tetraborate decahydrate, consist of
chains of edge-shared hexaaquasodium octahedra alternating with chains of discrete
[B 4 O 5 (OH) 4 ]
2− ions hydrogen-bonded to each other [13]. The tetraborate anion
consists of an oxygen-boron core with two tetrahedral and two trigonal boron atoms
reminiscent of bicyclo [3.3.1] nonane, where the bridgeheads correspond to the
tetrahedral boron atoms. The structure can therefore be reformulated as
Na 2 [B 4 O 5 (OH) 4 ] ⋅ 8H 2 O. The structure was refined by neutron diffraction [14].
The determination of the hydrogen atom positions indicate that the ion has C 2
symmetry [15]. The oxo atoms of the anion accept hydrogen bonds either from the
aquasodium chains (cross-chain) or from the hydroxyl groups (in-chain), and the
hydroxyl group donate cross-chain hydrogen-bonds to the water oxygen atoms.
Sodium tetraborate “pentahydrate” [16] (the mineral tincalconite, 1:2:4.667) is a
reversible dehydration product of the decahydrate. The [B 4 O 5 (OH) 4 ]
2− ions persist
in this structure, however, the hydroxyls of the tetrahedral boron atoms are now
directly coordinated to the sodium atoms. The structure can therefore be reformulated as Na 2 [B 4 O 5 (OH) 4 ] ⋅ 3H 2 O. The hydroxyls of the trigonal boron hydrogen
bond to the oxo group between the two tetrahedral boron atoms. Upon refinement,
incomplete occupancy of the water sites was found, corresponding to the formula
Na 2 [B 4 O 5 (OH) 4 ] ⋅ 2.667H 2 O [17]. The disorder was also confirmed at room
temperature [18]. It was suggested that sodium tetraborate tetrahydrate (the mineral
kernite, 1:2:4) was similar to borax in structure [19]; however this was shown to be
incorrect, as the boron units formed an infinite chain of six-membered rings
comprised of [B 4 O 6 (OH) 2 ] n
2n− [20, 21]. A later redetermination also included an
electron population analysis [22]. The monohydrate consists of chains of
108
C. C. Pye