Zwei Dodekahydro‐closo‐Dodekaborate mit Lone‐Pair‐Kationen der 6. Periode im Vergleich: Tl2[B12H12] und Pb(H2O)3[B12H12]·3H2O

During the reaction of an aqueous solution of (H₃O)₂[B₁₂H₁₂] with Tl₂CO₃ anhydrous thallium(I) dodecahydro‐closo‐dodecaborate Tl₂[B₁₂H₁₂] is obtained as colorless, spherical single crystals. It crystallizes in the cubic system with the centrosymmetric space group Fm$bar{3}$ (a = 1074.23(8) pm, Z = 4) in an anti‐CaF₂ type structure. Four quasi‐icosahedral [B₁₂H₁₂]^2– anions (d(B–B) = 180–181 pm, d(B–H) = 111 pm) exhibit coordinative influence on each Tl⁺ cation and provide a twelvefold coordination in the shape of a cuboctahedron (d(Tl–H) = 296 pm). There is no observable stereochemical activity of the non‐bonding electron pairs (6s² lone pairs) at the Tl⁺ cations. By neutralization of an aqueous solution of the acid (H₃O)₂[B₁₂H₁₂] with PbCO₃ and after isothermic evaporation colorless, plate‐like single crystals of lead(II) dodecahydro‐closo‐dodecaborate hexahydrate Pb(H₂O)₃[B₁₂H₁₂] · 3H₂O can be isolated. This compound crystallizes orthorhombically with the non‐centrosymmetric space group Pna2₁ (a = 1839.08(9), b = 1166.52(6), c = 717.27(4) pm, Z = 4). The crystal structure of Pb(H₂O)₃[B₁₂H₁₂] · 3H₂O is characterized as a layer‐like arrangement. The Pb²⁺ cations are coordinated in first sphere by only three oxygen atoms from water molecules (d(Pb–O) = 247–248 pm). But a coordinative influence of the [B₁₂H₁₂]^2– anions (d(B–B) = 173–181 pm, d(B–H) = 93–122 pm) on lead has to be stated, too, as three hydrogen atoms from three different hydroborate anions are attached to the Pb²⁺ cations (d(Pb–H) = 258–270 pm) completing their first‐sphere coordination number to six. These three oxygen and three hydrogen ligands are arranged as quite irregular polyhedron leaving enough space for a stereochemical lone‐pair activity (6sp) at each Pb²⁺ cation. Since additional intercalating water of hydration is present as well, both classical H–O^δ– ··· ^+δH–O‐ and unconventional B–H^δ– ··· ^+δH–O hydrogen bonds play a significant role in the stabilization of the entire crystal structure.