Structural Systematics of Rare Earth Complexes. XXII 4‐Methylpyridinium Salts of Diverse Complex Aqua/Chloro/Lanthanoid(III) Species

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 $P{\bar 1}$ series (mpyH) 2 [{(H 2 O) 3 Cl 3 Ln(μ‐Cl) (2|2) } 2 ], previously defined for the Ln = La–Nd members (those for La, Pr characterized by full structure determinations); (b) The triclinic $P{\bar 1}$ series (mpyH) 2 [(H 2 O) 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 C 2/ c series (mpyH) 2 [(H 2 O) 4 LnCl 3 ]Cl 2 , defined for the Ln = Er–Yb extrema (full structural characterizations for the Ln = Er, Tm, 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 [{(H 2 O) 3 Cl 3 La(μ‐Cl) (2|2) } 2 ]‐[{(H 2 O) 4 Cl 2 La(μ‐Cl) 2 La(OH 2 ) 2 Cl 2 (μ‐Cl) (2|2) } 2 ]Cl 4 ·6H 2 O·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 [(H 2 O) 5 LuCl 2 ]Cl 3 is defined by a full structure determination. (iii) For Ln = Y, (mpyH) 2 [(H 2 O) 7 YCl]Cl 4 is defined by a full structure determination; here, and in (ii), the complex component is cationic.