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In this paper are presented the syntheses, characterizations, and dynamic solution behaviors of three cis-[Ru(bpy)2(L)2] (bpy = 2,2'-bipyridine) complexes, 1-3, in which L represents the monodentate ligands 1-methylimidazole (MeIm), 1,2-dimethylimidazole (Me2Im), and 1-methylbenzimidazole (MeBim), respectively. Because of their different steric properties, these three monodentate ligands yield complexes that show quite different fluxional behaviors in solution. These behaviors are studied with several 1H NMR techniques at various temperatures between -95 and degrees C. The 1H NMR spectra of 1, which has the smallest monodentate ligand of the three used, indicate the complex to be in fast exchange (i.e., the imidazoles rotate around their Ru-N axes) at all recording temperatures. The sterically more demanding ligands, Me2Im and MeBim, in 2 and 3, respectively, are in fast exchange at 55 degrees C and in slow exchange at low temperatures, showing three different atropisomers: two head-to-tail (HT) isomers and one head-to-head (HH) isomer. The newly synthesized bidentate ligand 1,2-bis-(1-methyl-2-benzimidazolyl)ethane (mdbz) forms the complex cis-[Ru(bpy)2(mdbz)](PF6)2 (4), in which the two benzimidazole moieties are constrained and relatively fixed. The two tethered benzimidazoles in 4 cannot rotate around their Ru-N axes, and therefore 4 is a good model for the main HT isomer of 3.

A new iron(III) coordination compound exhibiting a two-step spin-transition behavior with a remarkably wide [HS-LS] plateau of about 45 K has been synthesized from a hydrazino Schiff-base ligand with an N,N,O donor set, namely 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol (Hmph). The single-crystal X-ray structure of the coordination compound {[Fe(mph)(2)](ClO(4))(MeOH)(0.5)(H(2)O)(0.5)}(2) (1) determined at 150 K reveals the presence of two slightly different iron(III) centers in pseudo-octahedral environments generated by two deprotonated tridentate mph ligands. The presence of hydrogen bonding interactions, instigated by the well-designed ligand, may justify the occurrence of the abrupt transitions. 1 has been characterized by temperature-dependent magnetic susceptibility measurements, EPR spectroscopy, differential scanning calorimetry, and (57)Fe Mossbauer spectroscopy, which all confirm the occurrence of a two-step transition. In addition, the iron(III) species in the high-spin state has been trapped and characterized by rapid cooling EPR studies.

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Die Reaktion der Mn‐, Co‐ und Ni‐nitrate in Wasser mit NH4NCS und den Liganden liefert die Komplexe (l).