Extended metal atom chains (EMACs) of five chromium or cobalt atoms: Symmetrical or unsymmetrical?

Structural studies of pentachromium and pentacobalt extended metal atom chain (EMAC) systems are presented in which the metal chains are helically wrapped by either the pentadentate dianion tripyridyldiamide (tpda) or its diethyl-substituted analogue bis(4-ethyl-2-pyridylamido)pyridine (etpda). The compound Cr-5(tpda)(4)(NCS)(2), (1) has alternating long and short Cr-Cr distances, contrary to recent reports that describe it as symmetrical with essentially equally spaced chromium atoms (Eur. J. Inorg. Chem., 1999, 1243; J. Phys. Chem. B, 2004, 108, 959). The linear Cr-5(10+) chain is composed of two Cr-2(4+) quadruply bonded units and an isolated high spin Cr-II unit. The new compounds Cr-5(etpda)(4)Cl-2 (2), [Cr-5(etpda)(4)FCl]PF6 (3), and Co-5(etpda)(4)(NCS)(2) (4) employ the etpda ligand as "insulation" around a central pentametal-atom "wire." Compound 3, like all other oxidized pentachromium compounds, has very disparate alternating short-long-short-long Cr-Cr distances, 2.032(3) Angstrom, 2.560(6) Angstrom, 1.873(5) Angstrom, and 2.509(4) Angstrom. Compound 4 shows a nearly uniform spacing of the Co atoms, although the outer Co-Co distances (2.279[4] Angstrom) are slightly longer than the inner ones (2.239[4] Angstrom).