Syntheses of (Me4N)2[(.mu.-EPh)6(MX)4] (M = Cd, Zn; E = S, Se; X = Cl, Br, I). Crystal and molecular structures of (Me4N)2[(.mu.-EPh)6(CdBr)4] (E = S, Se) and characterization of [(.mu.-EPh)6(CdX)4]2- in solution by cadmium-113 and selenium-77 NMR