Preparation and characterization of the 1:1 adducts of mercury(II) halides with N -benzyl- and N - p -tolyl-2-oxo-1-naphthylideneamine. The crystal and molecular structures of two isostructural di-μ-halo-bis[halo( N -benzyl-2-oxo-1-naphthylideneamine)mercury(II)] adducts (halo=chloro, bromo)