Synthesis, structural and electrochemical characterization of the nitrido-carbonyl cluster anion [Co13N2(CO)24]3−. The different redox propensity of the two isostructural families [Co13N2(CO)24]n− and [Co13C2(CO)24]m−

The anion [Co13N2(CO)24]3− (1) has been obtained by pyrolisis at 100 °C of [Co14N3(CO)26]3− (K+ salt) in buffered water solution (pH 11). The X-ray crystal structure ([NMe4]+ salt) revealed that the metal cage of the anion is composed by a three-layered polyhedron with the two nitrides in trigonal prismatic environments. Anion 1, with 178 cluster valence electrons (CVE), is isostructural with the carbido species [Co13C2(CO)24]3− (2, 176 CVE) and [Co13C2(CO)24]4− (3, 177 CVE). In acetonitrile solution 1 undergoes two reversible one-electron additions. Comparison with the dicarbido tetraanion 3 evidences that the two clusters do not display isoelectronic redox paths. Actually, [Co13N2(CO)24]n− undergoes the three-membered redox sequence (n=3–5) from 178 to 180 CVE, whereas [Co13C2(CO)24]m− undergoes the four-membered series (m=3–6), from 176 to 179 CVE. The different redox behaviour and the minor differences in the CoCo bond distances point out that carbide and nitride have different role in stabilizing the two compounds.