Hydrido-carbonyl chain clusters, synthesis, solid state structure, and solution behavior of the tetranuclear open cluster anions [Re4H(mu-H)(2)(CO)(17)](-) and [Re-4(mu-H)(CO)(18)](-)

The addition of [Re2H(CO)(9)](-) to the electronically unsaturated complex [Re-2(mu-H)(2)(CO)(8)] rapidly and selectively gives the anion [Re4H(mu-H)(2)(CO)(17)](-) (2), containing an open chain tetranuclear metal skeleton, as revealed by a single-crystal X-ray analysis of its [NEt4](+) salt. In the solid state the three metal-metal interactions display a staggered-eclipsed-staggered conformation, while in solution H-1 and C-13 NMR spectra have shown conformational freedom around the three Re-Re interactions and a dynamic process exchanging the two hydrides bound to the terminal H2Re(CO)(4) moiety, as well as the carbonyls trans to them (E-a = 48(1) kJ/mol). A windshield-wiper motion of the H2Re(CO)(4) fragment around the two trans diaxial carbonyls, analogous to that previously observed in the related anions [Re3H(mu-H)(CO)(13)](-) and [Re2H2(mu-H)(CO)(8)](-), is likely responsible for this exchange. The tetrametallic skeleton of the anion 2 in solution easily undergoes fragmentation to trinuclear species. Under CO atmosphere the clean formation of [ReH(CO)(5)] and [Re3H(mu-H)(CO)(13)](-) has been recognized. The anion 2 is formed (even if in lower yields) also by reaction of [Re2H2(mu-H)(CO)(8)](-) with ''Re-2(CO)(9)(THF)'', obtained by treatment of [Re-2(CO)(10)] with Me3NO in THF. A C-13 NMR investigation has clarified that such ''Re-2(CO)(9)(THF)'' reagent is indeed a mixture of three eq-[Re-2(CO)(9)L] species, containing THF, H2O, and, in a minor amount, NMe3, as labile L ligands. The reaction of the same eq-[Re-2(CO)(9)L] species with [Re2H(C0)(9)](-) affords in good yields the tetranuclear cluster anion [Re-4(mu-H)(CO)(18)}](-) (3). The single-crystal X-ray analysis of [NEt4]3 has revealed also in this case a Re-4 chain, with an all-staggered conformation, of idealized C-2 symmetry. The low-temperature C-13 NMR spectrum of the carbonyls has shown a higher symmetry in solution, suggesting conformational freedom around all of the Re-Re interactions.