Steric constraints in facultative ligands. The stereochemistry of cobalt(III), nickel(II), copper(II), and palladium(II) compounds of two homologous tetra-amines with terminal pyridyl residues

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Abstract
The relationship between the qualitatively defined steric effects in two homologous tetra-amine ligands, 1,6-bis-(2′-pyridyl)-2,5-diazahexane (Ia), 1,7-bis-(2′-pyridyl)-2,6-diazaheptane (Ib), and the stereochemistry of their compounds of cobalt(III), nickel(II), copper(II), and palladium(II) has been studied. Some copper(II) compounds of methyl derivatives of the ligands also are included. In agreement with the predictions from qualitative considerations of the steric effects which are here defined, trans-octahedral or ‘four-co-planar’ compounds are not favoured by these ligands, although copper(II) and palladium(II)‘four-co-planar’ species can be prepared and studied. In the octahedral compounds of cobalt(III) and nickel(II), only cis-species have been observed; where the configurations can be defined, as in cobalt(III), both cis-α- and cis-β-configurations of (Ia), but only cis-β-configurations of (Ib) have been characterised. Copper(II) gives a more complex stereochemistry, which appears to be dominated by co-ordination number five. Seventysix crystalline species are included in this study and structures are assigned from analytical data, i.r., vibrational, visible and ‘near-i.r.’, electronic, and 1H n.m.r. spectrometry, and X-ray diffraction techniques.

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