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 degreesC 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.