4-Methylpyridinium cations, mpyH(+), crystallized with complex aqua/chloro/lanthanoid(III) species for the gamut of the rare earth series, have 'domains of existence' defined for the following forms: (a) The triclinic P $(1) over bar $ series (mpyH)(2)[{(H2O)(3)Cl(3)Ln(mu-Cl)((2/2))}(2)], previously defined for the Ln= La-Nd members (those for La, Pr characterized by full structure determinations); (b) The triclinic P (1) over bar series (mpyH)(2)[(H2O)(3)LnCl(4)]Cl, previously defined for the Ln=Eu, Ho members by full structure determinations, is here augmented by the Ln=Nd, Sm examples (full structure determinations, the latter a new 'light' Ln extremum); (c) A new monoclinic C2lc series (mpyH)(2)[(H2O)(4)LnCl(3)]Cl-2, defined for the Ln=Er-Yb extrema (full structural characterizations for the Ln= Er, Tin, Yb members); the lanthanoid-containing entity (which, with the cations, exhibits some disorder) is a neutral molecule of a new type. For the Ln= Lu case, a fully ordered derivative monoclinic C-2 form has been obtained in a cell one half the size. Other types have also been characterized, thus: (i) For Ln=La, (mpyH)(8)[(H2O)(3)Cl3La(mu-Cl)((2/2))}2]-[{(H2O)(4)Cl2La(mu-Cl)(2)La(OH2)(2)Cl-2(mu-Cl)((2/2))}2]Cl-4 center dot 6H(2)-O center dot mpy has been defined by a full structural determination; the binuclear anion is similar to that in (a), albeit with some disorder, with the tetranuclear anion derivative of it. (ii) For Ln=Lu, (mpyH)(2)[(H2O)(5)LuCl2]Cl-3 is defined by a full structure determination. (iii) For Ln=Y, (mpyH)(2)[(H2O)(7)yCl]Cl-4 is defined by a full structure determination; here, and in (ii), the complex component is cationic.
