The crystal structures of two isomeric modifications of bis [p-((p-ethoxyphenyl)tellurio)]-bis(cyclopentadienylcarbonyliron) have been determined. Isomer I, C2gH28Fe2Fe204, crystallizes in the orthorhomibc space group Pbcn with a = 18.150 (9)A, b = 7.651 (4) A, c = 19.929 (14) A, V= 2767.6 A1 23, and Z = 4. Isomer II, C28H28Fe2Te2O4-0.8C6H12, crystallizes in the monoclinic space group P2\/c with a = 13.617 (4) A, b = 14.491 (3) A, c = 19.016 (6) Á, ß = 112.03 (2)°, V = 3478.4 A3, and Z = 4. Both isomers contain an -Fe-Te-Fe-Temacrocyclic ring which is slightly puckered about the axis joining the Fe atoms. The Fe and Te atoms in both compounds are found to have very similar coordination environments. Each Fe atom lies at the center of a distorted tetrahedron (82.78 (5)-128.6(8)°), four-coordination being completed by a carbonyl and a cyclopentadienyl ligand, while each Te atom lies at the apex of an irregular trigonal pyramid (93.80 (6)-108.9(2)e), three-coordination being completed by bonding to a p-ethoxyphenyl ligand. The isomerism exhibited by these compounds is geometric. Isomer I has its two Cp ligands cis disposed with respect to one another and the puckering of the Fe2Te2 ring, while on the opposite side of the ring are found both pairs of p-ethoxyphenyl and carbonyl ligands, also cis with respect to one another. Isomer II has each pair of carbonyl, cyclopentadienyl, and p-ethoxyphenyl ligands approximately trans. The 125Te and 57Fe Mdssbauer parameters of both isomers are discussed.

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Mössbauer data are reported for a number of heterocyclic compounds containing tellurium. The influence on the Mössbauer parameters of the constraints imposed by accommodating tellurium in these ring systems are discussed. The influence of the bulky aryl ligand 2-biphenylyl, C12H9—, on the Mössbauer parameters of diaryltellurium dihalides and aryltellurium trihalides has also been investigated.

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