Synthesis and Structure of Triphenylmethylphosphonium Tetrakis(tetrahydroborato)Aluminate, [Ph3MeP][Al(BH4)4], an Example of Eight-Coordinate Aluminum(III)

Structural and dynamic properties of tetrahydroborate complexes

“…222 The type of coordination of tetrahydroborate ligands in the metal complexes in question cannot be determined from the data of NMR spectroscopy; these data attest to equivalence of all the four protons in the BH 4 ligand. 145,147,148,157,161 The data of IR spectroscopy for alkali and alkaline earth metal complexes do not conform to any particular type of coordination of the tetrahydroborate groups either. 145,147,148,157 The structures of the tetrahydroborate complexes with the central d 0 -metal ions in the oxidation state +1 as well as complexes with the d n -metal central ions (n = 2 ± 10) cannot be explained in terms of the principle of the maximum occupancy of the coordination sphere.…”

Some tetrahydroborate derivatives of aluminium: crystal structures of dimethylaluminium tetrahydroborate and the α and β phases of aluminium tris(tetrahydroborate) at low temperature

“…The B᎐H t and B᎐H b bond lengths are very similar to those found in the [Al(BH 4 ) 4 ] Ϫ ion [0.95(5)-1.22(5) and 0.80(5)-1.07(5) Å, respectively]; 40 differences in the Al᎐H and Al ؒ ؒ ؒ B distances between the two species and the existence of less symmetrical Al(µ-H) 2 B bridges in [Al(BH 4 ) 4 ] Ϫ almost certainly reflect the greater steric congestion and distorted dodecahedral geometry peculiar to the anion. 40 The molecular structure of Al(BH 4 ) 3 ؒNMe 3 contains seven-co-ordinated aluminium, the NMe 3 ligand and six bridging hydrogens adopting a distorted pentagonal-bipyramidal geometry. 41 The Al ؒ ؒ ؒ B distances for this complex [2.23(1) Å] are similar to those observed for both phases of Al(BH 4 ) 3 .…”

“…Most obvious is the ability of BH 4 , but not CH 3 , to act as a bidentate ligand and thereby occupy two co-ordination sites at the metal; this allows Al(BH 4 ) 3 to retain its discrete molecular nature in the solid state, whereas 'Me 3 Al' dimerizes to give Me 6 Al 2 . 45 A polymeric structure for solid Al(BH 4 ) 3 featuring bis(bidentate) BH 4 groups would imply an eight-co-ordinate geometry at the aluminium centre, and, although such a geometry finds precedent in the [Al(BH 4 ) 4 ] Ϫ anion, 40 there is little suggestion of any significant secondary interaction in either the α or the β phase of Al(BH 4 ) 3 . Progressing from Me 6 Al 2 to [Me 2 AlBH 4 ] witnesses a change from methyl-to tetrahydroborate-bridged aggregation.…”

Complexation of Ammonia Boranes with Al3+

“…The Al 3+ cation is linked via BH 2 edges and hence adopts a distorted tetrahedral coordination made up of borohydride groups, while the AlH 8 polyhedron has the shape of a snub disphenoid, similar to Al in Al­(BH 4 ) 3 ·AB or Mg in Mg­(BH 4 ) 2 . ,, The Al···B distances with the BH 4 – ions are in the narrow range of 2.22–2.24 Å, similar to 2.21–2.23 Å in the two Al­(BH 4 ) 3 ·NH 3 BH 3 polymorphs. This distance is nearly identical with the Al···B distances of 2.22–2.26 Å in M­[Al­(BH 4 ) 4 ] (M = Li + , Na + , K + , NH 4 + , Rb + , Cs + ) and [Ph 3 MeP]­[Al­(BH 4 ) 4 ], where the Al 3+ cation is also coordinated to eight H atoms. − The interatomic Al···B contact involving the MeAB’s BH 3 group is slightly longer (2.34 Å) than the distances to the BH 4 – anions, fully consistent with the elongation in Al­(BH 4 ) 3 ·AB (2.31 Å). However, they remain much shorter than the metal–boron distances in metal borohydride–AB complexes, namely, 2.63–2.92 Å in (LiBH 4 ) 2 ·AB, LiBH 4 ·AB, and Ca­(BH 4 ) 2 ·(AB) 2 . , Al–H bond distances vary accordingly: for the BH 4 – groups, they range from 1.76(2) to 1.80(2) Å, similar to those in Al-based complex hydrides, − and vary from 1.89(1) to 1.96(1) Å, where the AB’s −BH 3 group is involved …”