Structural effects of the lone pair on lead(II), and parallels with the coordination geometry of mercury(II). Does the lone pair on lead(II) form H-bonds? Structures of the lead(II) and mercury(II) complexes of the pendant-donor macrocycle DOTAM (1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane)

The synthesis and structures of [Pb(DOTAM)](CIO4)(2).4.5H(2)O (1) and [Hg(DOTAM)](CIO4)(2).0.5CH(3)OH.1.5H(2)O (2) are reported, where DOTAM is 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane. Compound 1 is triclinic, space group P (1) over bar, a = 12.767(3) Angstrom, b = 13.528(2) Angstrom, c = 18.385(3) Angstrom, alpha = 101.45(2)degrees, beta = 93.32(2)degrees, gamma = 90.53(2)degrees, Z = 4, R = 0.0500. Compound 2 is monoclinic, space group Cc, a = 12.767(3) Angstrom, b = 13.528(2) Angstrom, c = 18.385(3) Angstrom, beta = 101.91(2)degrees, Z = 4, R = 0.0381. The Pb(II) ion in 1 has an average Pb-N = 2.63 Angstrom to four N-donors from the macrocyclic ring, and four O-donors (average Pb-O = 2.77 Angstrom) from the amide pendant donors of the macrocycle, with a water molecule placed with Pb-O = 3.52 Angstrom above the proposed site of the lone pair (Lp) on Pb. The Hg(II) in 2 appears to be only six-coordinate, with four Hg-N bond lengths averaging 2.44 Angstrom, and two Hg-O from pendant amide donors at 2.41 Angstrom. The other two amide donors appear to be noncoordinating, with Hg-O distances of 2.74 and 2.82 Angstrom. A water situated 3.52 Angstrom above the proposed site of the lone pair on Pb(II) in 1 is oriented in such a way that it might be thought to be forming a Pb-Lp...H-O-H hydrogen bond. It is concluded that that this is not an H-bond, but that the presence of the lone pair allows a closer approach of the hydrogens to Pb than would be true otherwise. The structural analogy in the VSEPR sense between Pb(II), which has the 5d(10)6S(2) outer electron structure, and the Hg(II) ion, which has the 5d(10) structure, is examined. The tendency of Hg(II) toward linear coordination, with two short Hg-L bonds (L = ligand) at 1800 to each other, and other donor groups at roughly 900 to this and at much longer bond distances, is paralleled by Pb(II). One of the short Hg-L bonds is replaced in the Pb(II) structures by the lone pair (Lp), which is opposite the short Pb-L bond, or in some cases 2-4 shorter Pb-L bonds.