Tetraphenylphosphonium octahydrotriborate

The structure of the title salt, C24H20P+·H8B3 −, at 120 (2) K has triclinic (P1) symmetry with an unusual Z = 5, although there is pseudosymmetry observed with the tetraphenylphosphonium cations exhibiting I symmetry. One of the anions is disordered over two sets of sites with refined occupancies of 0.478 (11) and 0.522 (11).

The structure of the title salt, C 24 H 20 P + ÁH 8 B 3 À , at 120 (2) K has triclinic (P1) symmetry with an unusual Z = 5, although there is pseudosymmetry observedwith the tetraphenylphosphonium cations exhibiting I4 symmetry. One of the anions is disordered over two sets of sites with refined occupancies of 0.478 (11) and 0.522 (11).

Experimental
Hence the requirement to solve in P1.

Refinement
The initial data collection strategy was for a body-centred tetragonal crystal system (a= 27.593 (2)Å, c= 7.031 (1)Å), with a calculated Z of 10. This data was solvable to a reasonable degree in I4, with an R1=0.124, Z'=1.25, but terrible disorder of the two symmetry inequivalent octahydrotriborate anions. This was not helped by the fact one lay right on the 4-fold rotoinversion axis, for which it is physically impossible to obtain a non-disordered triangle. To help prove likely composition of the compound, this was enough information at the time. To fully prove the structure, further investigations were finally carried out which included lowering the symmetry of the crystal system and space group. It was discovered that when the structure was solved in the reduced triclinic unit cell instead of the original body-centred cell, almost all the disorder disap-supplementary materials sup-2 peared and that without having to use any thermal restraints the displacement parameters were also reasonable. Although the raw data was reintegrated as triclinic to obtain the necessary weak reflections and differing intensities which broke the body-centred tetragonal reflection rules, there was still an overall lack of data since the strategy for the data collection had been based on the tetragonal unit cell (77.6% complete against 99.7% complete out to 0.77Å resolution). This does present a few problems with regard to the overall quality of the structure found. With many fewer equivalent reflections collected, there will be consequential drop-off in the accuracy of the scaling of the data and the absorption correction carried out. This is still enough information for the clear proof of structure, however there is a very slight (but still noticeable) increase in the e.s.d.s of all the bond lengths, angles and the displacement parameters. These still lie within acceptable limits but are all slightly larger than normal for a low temperature crystal structure.
The hydrogen atoms of the phenyl rings were placed at calculated positions using the standard riding model in SHELX for aromatic hydrogen atoms. The terminal (non-bridging) hydrogen atoms of the octahydrotriborate anion was also placed at calculated positions using the standard riding model, this time for methylene hydrogen atoms. The bridging hydrogen atoms were located whenever possible from the difference map. When this was not possible the three boron-boron distances of the triangle were checked and a hydrogen atom was initially placed at the mid-point of the two shorter B-B bonds. No matter how they were located, all the bridging hydrogen atoms were restrained by 3 distance restraints, which were based on average distances for these compounds already found in the Cambridge Structural Database. These distances were 1.16 (6)Å from the two boron atoms it was bridging across and 2.30 (6) Å from the opposite boron atom. These constitute most of the restraints used in the structure.

Special details
Experimental. The data was collected based on a strategy for a body-centred tetragonal crystal system. Only during much later analysis was it noticed that it should be integrated using the primitive crystal system as presented.
SADABS was used to perform the Absorption correction, Estimated minimum and maximum transmission: 0.6028 0.7455 The given Tmin and Tmax were generated using the SHELX SIZE command supplementary materials sup-4 Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
One of the octahydrotriborate anions was disordered over two sites. Due to resulting low expected electron density for each boron atom in this anion, they were left isotropic.
For each octahydrotriborate anion, the two bridging hydrogen atoms were located using a combination of either from the difference map and/or based on the boron-boron distances. They were then restrained to suitable distances from the boron atoms (1.16 (6)Å for the two adjacent and 2.30 (6)Å for the one opposite respectively).