[P₃Se₇]³⁻: A Phosphorus-Rich Square-Ring Selenophosphate

The new compound Cs₁₀P₈Se₂₀ features the heterotetracyclic [P₃Se₇]³⁻ anion, a phosphorus-rich four-membered-ring species that possesses a P−P−P unit and formally P³⁺ and P⁴⁺ centers. It crystallizes in the orthorhombic space group Pnnm with a = 26.5456(7) Å, b = 8.0254(2) Å, c = 11.9031(4) Å, and Z = 2 at 100(2) K. The cyclic anion is cocrystallized with a [P₂Se₆]⁴⁻ anion. Electronic absorption, Raman, Fourier transform infrared, and solid-state ³¹P NMR spectroscopy studies of Cs₁₀P₈Se₂₀ are reported.