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-

Abstract

The alkali metal boron compounds M3[BX2] 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 (M3[BP2]) or plates (K2Na[BP2]) and yellow-red prismatic crystals (M3[BAs2], Cs3[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 structure type); Na3[BP2] (P21/c (No. 14); Z = 4, β-Li3[BN2] type), M3[BX2] with M = K, Rb, Cs and X = P, As and Cs3[P-B-As] (C2/c, (No. 15); Z = 4, K3[BP2] 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.