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The temperature dependence of the bond lengths in the high temperature phase of [Cu(L)2][BF4]2 (L = 2,6-dipyrazol-1-ylpyridine) may be interpreted satisfactorily using a model of dynamic, Jahn–Teller (JT) vibronic coupling. The geometries of the three complexes in the asymmetric unit of the low temperature phase may be reproduced using parameters similar to those of the high temperature phase, but with significantly larger orthorhombic components of the strain interactions. The axial components of the strain for all the complexes in both phases are negative in sign and large in magnitude, and this is thought to reflect both the stronger σ-bonding power of pyridine compared with pyrazole and physical constraints of the amine ligand. A previous interpretation of the thermal behaviour of the high temperature phase in terms of a conformational equilibrium is re-evaluated.

The crystal structures of [Cu3(bipyam–H)4Cl2]·H2O (1) and [Cu3(bipyam–H)4Br2]·H2O (2) where bipyam–H = bis(2-pyridyl)amide, have been determined by X-ray analysis, in the orthorhrombic space group Pnn2: (1), a= 14.092(3), b= 12.895(3), (c)= 11.190(2)A, Z= 2, and R= 0.032 for 2 453 observed and 2 029 unique reflections; (2), a= 14.186(3), b= 13.040(3), c= 11.313(2)A, Z= 2, and R= 0.043 for 1 574 observed and 1 465 unique reflections. The two structures are isomorphous with near isostructural [Cu3(bipyam–H)4X2] units in special positions of two-fold symmetry and a non-co-ordinated water molecule. The Cu3N12X2 chromophores involve nearly linear Cu3 units, Cu–Cu–Cu 178.4°(mean), terminated by the two halide anions. The four separate bipyam–H ligands act as tridentate ligands, involving co-ordination to the three separate copper(II) ions, with Cu–Cu distances of 2.471(1) and 2.468(1)A, for (1) and (2), respectively. If the Cu–Cu separations are ignored, the central Cu atom in both structures involves a four-co-ordinate rhombic coplanar CuN4 chromophore generated by the central amido nitrogens of the four bipyam–H ligands. The two terminal Cu atoms involve a square-based pyramidal CuN4X chromophore, generated by the terminal pyridine nitrogens of the four bipyam–H ligands and an axial halide anion. An average dihedral angle of 48° is involved between the planes of the pyridine rings of the individual bipyam–H ligands, which results in a spiral configuration of the [Cu3(bipyam–H)4X2] units. The spin-only magnetic moment of complex (1) is ca. 1.40 B.M. per Cu atom, consistent with antiferromagnetic coupling between the copper(II) atoms of the trimer. Both complexes are e.s.r. silent, again consistent with strong antiferromagnetic coupling. The electronic spectra of (1) and (2) have a band maximum at 15 500 cm–1, with a high-energy shoulder at 19 230 cm–1, consistent with the two different stereochemistries present.

In its complex with CuI, dipyrimidin-2-yl disulphide is centrosymmetric and planar with a C–S–S–C torsion angle of 180° and a long S–S bond, 2·113(1)A.

The X-ray crystal structures of [Cu(bipy)2(ONO)][NO3](bipy =2,2′-bipyridyl) are reported at 298 and 165 K; significant differences occur which are consistent with a fluxional behaviour of the CuN4O2 chromophore, and enable the underlying static stereochemistry to be predicted and rationalised using the i.r. spectrum of the nitrito-group and the electronic reflectance spectrum of the CuN4O2 chromophore.

The crystal structure of [Cu(phen)2(O2CMe)][ClO4]·2H2O (1), [Cu(phen)2(O2CMe)][NO3]·2H2O (2), and [Zn(phen)2(O2(CMe)][BF4]·2H2O (3) have been determined by X-ray analysis. Complex (1) crystallises in the monoclinic space group P2/c with a= 9.671(3), b= 8.282(3), c= 17.595(4)A, β= 109.63(2)°, and Z= 2; (2) crystallises in the triclinic space group P with a= 14.728(2), b= 10.499(2), c= 8.603(2)A, α= 104.19(5), β= 83.97(5), γ= 96.81(5)°, and Z= 2;(3) crystallises in the monoclinic space group P2/c with a= 9.659(2), b= 8.262(2), c= 17.651(2)A, β= 109.06(5)°, and Z= 2. The MN4O2 chromophore in all three complexes has a cis-octahedral stereochemistry. In (1) and (3) the metal atoms have C2 symmetry and in (1) there is a marked cis distortion of the Cu–O distances. In (2) a distortion of the CuN4O2 chromophore towards a square pyramidal (4 + 1 + 1*) stereochemistry is present. The polycrystalline e.s.r. spectra of (1) and (2) and of 10% copper(II)-doped (3) are temperature variable, consistent with a fluxional CuN4O2 chromophore stereochemistry. The temperature variability of the single-crystal e.s.r. spectra of (1), (2), and copper(II)-doped (3) is restricted to the CuN2O2 plane, consistent with two-dimensional fluxional behaviour. The electronic reflectance spectra of (1) and (2) involve two peaks at ca. 10 000 and ca. 14 000 cm–1 and relate to the underlying static CuN4O2(4 + 1 + 1*) chromophore stereochemistry.

The X-ray crystal structures of four cation-distortion isomers of the [Cu(Phen)2(O2CMe)]+(phen = 1,10-phenanthroline) cation are reported and shown to contain the cis-octahedral CuN4O2 chromophore; X-ray and e.s.r. data are consistent with a fluxional CuN4O2 structure, which is best described as a psuedo cis-distorted octahedral copper(II) stereochemistry in three of the complexes, and consequently these structural variations can be related to the dynamic pseudo-Jahn-Teller effect.