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-tetraaza

The synthesis and structures of [Pb(DOTAM)](ClO4)2·4.5H2O (1) and [Hg(DOTAM)](ClO4)2·0.5CH3OH·1.5H2O (2)
are reported, where DOTAM is 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane. Compound 1
is triclinic, space group P1̄, a = 12.767(3) Å, b = 13.528(2) Å, c = 18.385(3) Å, α = 101.45(2)°, β = 93.32(2)°,
γ = 90.53(2)°, Z = 4, R = 0.0500. Compound 2 is monoclinic, space group Cc, a = 12.767(3) Å, b = 13.528(2)
Å, c = 18.385(3) Å, β = 101.91(2)°, Z = 4, R = 0.0381. The Pb(II) ion in 1 has an average Pb−N = 2.63 Å to
four N-donors from the macrocyclic ring, and four O-donors (average Pb−O = 2.77 Å) from the amide pendant
donors of the macrocycle, with a water molecule placed with Pb−O = 3.52 Å 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 Å,
and two Hg−O from pendant amide donors at 2.41 Å. The other two amide donors appear to be noncoordinating,
with Hg−O distances of 2.74 and 2.82 Å. A water situated 3.52 Å 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 5d106s2 outer electron structure, and the Hg(II) ion, which has the 5d10 structure, is examined. The tendency
of Hg(II) toward linear coordination, with two short Hg−L bonds (L = ligand) at 180° to each other, and other
donor groups at roughly 90° 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.