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 M-3[BX2] With X = P, As are synthesized fr om the alkali metals M and the binary components MX or M4X6 and EX in seale d steel ampoules (phosphides) or niobium ampoules (arsenides) at 1000 K. Th e compounds are obtained as bright yellow prisms (M-3[BP2]) Or plates (K2Na [BP2]) and yellow-red prismatic crystals (M-3[BAs2], Cs-3[BPAs]) which are very sensitive against oxidation and hydrolysis. Three different structure types are formed, namely K2Na[BP2] (C2/m (No. 12); Z = 4; a new mC24 struct ure type); Na-3[BP2] (P2(1)/c (No. 14); Z = 4, beta-Li-3[BN2] type), M-3[BX 2] with M = K, Rb, Cs and X = P, As and Cs-3[P-B-As] (C2/c, (No. 15); Z = 4 , K-3[BP2] type). The bond lengths of the linear [BX2](3-) anions are hardl y 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 ex istence of unmixed and mixed units [P-B-P](3-), [As-B-As](3-) and [P-B-As]( 3-) with D-infinity h and C-infinity v symmetry, respectively. The valence force constants f(B-X) and the corresponding Siebert bond orders, calculate d from the frequencies, are discussed and compared with those of the isoele ctronic anions and molecules.