Interaction of Methylmercury(II) with N-Substituted Pyrazoles. Sigma-Donor Ability of Pyridines, Imidazoles, and Pyrazoles

N-Substituted pyrazoles (L) form complexes [MeHgL]NO, for which 2J(1H-199Hg) decreases with increasing basicity of L in a very similar manner to that observed for closely related N-substituted imidazole complexes. The relationships differ from that for complexes of pyridines and suggest that for ligands of similar log KH (pKa of LH+ in 50% dioxane-water) the N-substituted imidazoles and pyrazoles are better sigma donors toward MeHg(II) than pyridines. The complexes [MeHgL]NO3 [L = N-(2-pyridyl)pyrazole and bis(N-pyrazoly1)methane] have the ligands chelated to MeHg(II) when in methanol.