The solvent-bridged caesium phosphide {[CsPH(η
6-2,4,6-
t
Bu3C6H2)]2(η
3-toluene)0.5}
x
, catena-[(μ-η3-toluene)-bis[caesium(2,4,6-tri-tert-butylphenylphosphide)]], undergoes a reversible solid-state, order–disorder phase transition characterized by the doubling of the unit-cell volume at low temperature achieved by doubling one unit-cell vector. The unit-cell parameters at 293 (2) K (form A) are: a = 11.147 (4), b = 14.615 (4), c = 14.806 (5) Å, α = 70.57 (3), β = 71.85 (3), γ = 72.93 (2)°, V = 2112.5 (12) Å3, Z = 2, ρcalc = 1.362 g cm−3, R
1 = 0.0513 for 5462 reflections, wR
2 = 0.0947 for all data. The unit-cell parameters at 173 (2) K (form B) are: a = 14.6241 (3), b = 14.7393 (3), c = 22.0720 (4) Å, α = 72.2117 (7), β = 73.3659 (8), γ = 70.2953 (7)°, V = 4174.8 (2) Å3, Z = 4, ρcalc = 1.379 g cm−3, R
1 = 0.0405 for 14 010 reflections, wR
2 = 0.1326 for all data. With a minor change, the key structural features discussed previously for form A [Rabe et al. (1998). Inorg. Chem.
37, 4235–4245] remain unchanged. The η
3-toluene ligand is observed to be disordered at 293 (2) K and ordered at 173 (2) K, with the order–disorder phase transition occurring at approximately 278 (2) K.
