Darstellung,Kristallstrukturen und Schwingungsspektren von Verbindungen mit den linearen Dipnictidoborat(3–)‐Anionen [P–B–P]3–, [As–B–As]3– und [P–B–As]3–

Synthesis, Crystal Structure, and Vibrational Spectra of Compounds with the Linear Dipnictidoborate (3–) Anions P–B–P]^3–, As–B–As]^3–, and P–B–As]^3– The alkali metal boron compounds M₃BX₂] with X = P, As are synthesized from the alkali metals M and the binary components MX or M₄X₆ and BX in sealed steel ampoules (phosphides) or niobium ampoules (arsenides) at 1000 K. The compounds are obtained as bright yellow prisms (M₃BP₂]) or plates (K₂NaBP₂]) and yellow‐red prismatic crystals (M₃BAs₂], Cs₃BPAs]) which are very sensitive against oxidation and hydrolysis. Three different structure types are formed, namely K₂NaBP₂] (C2/m (No. 12); Z = 4; a new mC24 structure type); Na₃BP₂] (P2₁/c (No. 14); Z = 4, β‐Li₃BN₂] type), M₃BX₂] with M = K, Rb, Cs and X = P, As and Cs₃P–B–As] (C2/c, (No. 15); Z = 4, K₃BP₂] type). The bond lengths of the linear BX₂]^3– anions are hardly changed and correspond to a Pauling bond order PBO = 1.9 (d(B–P) = 176.7–177.1 pm; d(B–As) = 186.5–188.0 pm). The vibrational spectra confirm the existence of unmixed and mixed units P–B–P]^3–, As–B–As]^3– and P–B–As]^3– with D_∞h and C_∞v symmetry, respectively. The valence force constants f(B–X) and the corresponding Siebert bond orders, calculated from the frequencies, are discussed and compared with those of the isoelectronic anions and molecules.