A Short Pb.Pb Separation in the Polymeric Compound Bis(pyrrolidinecarbodithioato)lead(II) and a Conformational Pathway Interconversion for the "Pb(II)S(4)" Framework.

The X-ray structure of bis(pyrrolidinecarbodithioato)lead(II), Pb(II)(pdtc)(2), C(10)H(16)N(2)S(4)Pb, shows the metal to be on a crystallographic 2-fold axis. The primary Pb-S bonds are equal, 2.89 Å; this is a unique feature of this compound compared to other Pb(II) dithiocarbamate structures. The crystal structure consists of polymeric Pb(pdtc)(2) units stacked along the c axis in two antiparallel columns; successive Pb(pdtc)(2) units are staggered. There are weaker Pb-S bonding interactions with the unit above (secondary Pb-S bonds) such that the coordination polyhedron formed by the eight S atoms bound to the Pb atom is a distorted square antiprism. Successive antiprisms share the pyramidal base, and the Pb.Pb distance between neighboring Pb(pdtc)(2) units is 3.886(1) Å. We analyze the structures of other PbS(4) compounds and find that the Pb lone pair is important in defining their geometry. The large variety of Pb configurations results from peripheral changes in the ligand. This is due to a compromise between the efficient use of space in the crystal and the tendency of lead to have different coordination numbers. These configurations can be described by a pathway of conformational interconversion from a 4-sided pyramidal geometry to an octahedral system. The title compound crystallizes in the orthorhombic space group Pccn with z = 4; cell constants are a = 13.839(14) Å, b = 14.058(16) Å, c = 7.763(6) Å.