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The crystal structure of tetrakis(cytosine)copper(rI) perchlorate dihydrate has been determined. All the hydrogen atoms were obtained from Fourier-difference synthesis. The geometry around copper is a bicapped octahedron (4 + 2 + 2*). The adjacent cytosine rings are oriented head-to-tail with respect to each other and are roughly at right angles to the co-ordination plane. The exocyclic 0x0 groups form an interligand, intracomplex hydrogen-bonding network above and below the co-ordination plane with the exocyclic amino groups of alternate cytosine bases. The EPR and electronic spectra are consistent with the retention of the solid-state structure in solution. The steric effect of the C(2)=0 group of cytosine is offset by the presence of the intracomplex hydrogen-bonding network. The trend in El values of Cu"-Cu' couples for 1 : 4 complexes of cytosine, cytodine, pyridine, 2-methylpyridine and N-methylimidazole suggests that both steric effects and n-delocalization in imidazole and pyridine ligands and the steric effect of C(2)=0 in pyrimidine ligands are important in stabilising Cu' relative to Cu".

The linear quadridentate ligand N,N'-bis(benzimidazoI-2-ylethyl)ethane-l,2-diamine (L') and its 1methylbenzimidazole analogue (L2) and homologues form 1 : 1 complexes with Cu(CIO,),; L' also forms complexes of the types CuL'X, where X = NO,, PF,, Br or CI and CuL'(X)Y where X = CI or Br and Y = CIO, or Br. Deep blue CuL1Br,*2H20 crystallizes in the monoclinic space group C2/c with Z = 4,

A tripod ligand possessing two pyridine moieties and a phenolato group as pendants forms a mononuclear complex with an axial copper(ii)-phenolate co-ordination in a square-pyramidal environment.* For the polycrystalline sample at room temperature, gll = 2.269, go = 2.073, A, = 13 x 1 V cm-'. In dmf at 77 K, g l l = 2.244, A I , = 179 x 10-4cm-1,g, = 2.070, A, = 33 x l p c m -' .

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