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The crystal and molecular structure of [(Fe salen),0],2py (salen = NN'-ethylenebis(salicylideneiminato, py = pyridine) has been determined by single-crystal X-ray analysis. The crystallographic asymmetric unit comprises one dimeric molecule and two non-bonded pyridine molecules in a triclinic cell of dimensions, a = 12.73, b = 13-73, c = 13-92 8, cc = 11 8.75, p = 74.1 5, y = 11 6.1 7" ; space group PT. The iron atoms are joined by a single oxygen atom and both are five-co-ordinate with a distorted square pyramidal geometry. The mean Fe-O(bridge) bond length is 1-80 a and the bridging angle Fe-O-Fe is markedly bent a t 139". The properties of the dimeric molecule are compared with those of other oxo-bridged iron(ttl) and manganese(li1) dimers and also with the closely related [Fe salen CI], dimer and Fe salen CI monomer.

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'-pyridyI)-2,6-diazaheptane (Ib), and the stereochemistry of their compounds of cobalt(lt1). nickel(ll), copper(1i). and palladium(ll) has been studied. Some copper(l1) 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(i1) and palladium(ll) ' four-co-planar ' species can be prepared and studied. In the octahedral compounds of cobalt(ll1) and nickel(11). only cis-species have been observed ; where the configurations can be defined, as in cobalt(tii), both cis-uand cis-@-configurations of (Ia), but only cis-pconfigurations of (Ib) have been characterised. Copper(l1) 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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