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Nature © Macmillan Publishers Ltd 19988 letters to nature NATURE | VOL 394 | 9 JULY 1998 159 pressure are consistent with a maximum T c Ϸ 100 K, decreased by reductions in the A-cation size and disorder at the A (and perhaps other) sites. The quadratic dependence of T c upon radius in equations (1) and (2) shows that lattice strains are responsible for these variations. j in equation (1) and (r m A Ϫ r A ) in equation (2) are complementary measures of the displacements of adjacent oxygen atoms due to A-cation disorder and size which result in deformations and rotations of the linked CuO 6 octahedra.The changes of transition temperatures with j 2 reflect their sensitivity to lattice effects and give a measure of the accompanying lattice distortion. In our (Ln 0.925 M 0.075 ) 2 CuO 4 series with r A ¼ 1:223Å, the purely structural orthorhombic-tetragonal change, which is a typical soft mode transition at which a static lattice distortion occurs 16 , has an average dT ot =dj 2 Ϸ 38;000 KÅ Ϫ 2 . dT m =dj 2 Ϸ Ϫ 21;000 KÅ Ϫ 2 for the metal-insulator transition in the (Ln 0.7 M 0.3 )MnO 3 manganites is smaller but comparable in magnitude to the former value, reflecting the formation of large static Jahn-Teller distortions that accompanies the localization of carriers at T m (ref. 4,17).

The preparation and spectral and magnetic properties are reported for the complexes of the type Co(imidazole),X, and Ni(imidazole),X, (X = CI, Br, I, or NO,) and Cu(imidazole),X, (X = CI or NO,). These compounds have octahedral or distorted octahedral structures whereas the compounds Co(imidazole),CI, and di-imidazolatocobalt(l1) are tetrahedral. The " inner " complex di-imidazolatonickeI(I1) has a square-planar structure. The thiazole complexes Ni(thiazole),CI,, Co(thiazole),CI,, and Cu(thiazole),CI, are described and are shown to have essentially octahedral structures.IMIDAZOLE is of considerable interest as a ligand in that its presence in many biological systems (for example in the histidyl residue of proteins) provides a potential binding site for metal ions. A thorough knowledge of the co-ordinating properties of imidazole is therefore necessary to understand more fully the role of metal ions in such systems. A limited amount of work has been published on imidazole and related ligands, and a more exhaustive study of the complexing properties of such compounds, both in solutions and in the solid state, is in progres2 As part of this work we have isolated and investigated the complexes formed between imidazole or thiazole and some copper(II), cobalt(n), and nickel(I1) See rcfcrences in " Stability Constants of hletal-ion Com-W. Inoug. Chem., 1965, 4, 1613.Chein. Soc., 1964, 5194. 174.compounds. Although several compounds of transition metals with substituted imidazoles have been prepared,3-1° reports of complexes with imidazole itself have been confined to the preparation of the " inner " complexes of silver, nickel, zinc, and copper,l1'l2 and a brief discussion of the structure of di-imidazolatocopper(r1) .13Nickel Complexes .-Complexes of the type Ni(imidazole),X, (X = C1, Br, I, or NO,) were readily obtained by precipitation with ether from ethanolic solutions. Attempts to prepare complexes containing a ligand-tometal ratio of less than 6 : 1 were not successful by this method although pyridine readily forms complexes of

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