Hydrido-carbonyl rhenium clusters with a square geometry of the metal core. Synthesis and X-ray characterization of the novel [Re-4(mu-H)(3)(CO)(16)](-) anion

Three and tetranuclear ring clusters have been obtained by treatment of [Re -2(CO)(8)(THF)(2)] with carbonyl-rhenates containing two terminal hydrides. The reaction with [ReH2(CO)(4)](-) provided a selective route to the previ ously known [Re(mu -H)(2)(CO)(12)](-) triangular cluster anion 1. The react ion with [Re2H2(mu -H)(CO)(8)](-) gave the novel [Re-4(mu -H)(3)(CO)(16)](- ) anion 2. containing a rare example of a puckcred-square metal cluster. Pr otonation of 1 is known to afford the neutral [Re-3(mu -H)(3)(CO)(12)] spec ies 3. Analogously the reaction of 2 with a strong acid afforded the previo usly known square metal clusters [Re-4(mu -H)(4)(CO)(16)] 4. The reaction c ould not be reversed by treatment with bases. Photolysis of 4 gave the unsa turated complex [Re-2(mu -H)(2)(CO)(8)] 5: this is the reverse of the dimer ization reaction, that in THF at room temperature produces 4 from 5. Therma l treatment (reflux in cyclohexane for 24 hi left 4 almost unchanged. A sin gle crystal X-ray analysis of [NEt4]2 showed a s/e/s/s (e = eclipsed,s = st aggered) conformation of the Re( CO)4 units, leading to a puckered geometry of the ring, at variance with the square-planar geometry of 4 tall eclipse d). Two of the three hydrides of 2 have been located as bridging the Re Re edges from inside the metal ring, as previously observed in 1. Density func tional computations indicated a puckered conformation as the most stable fo r both 2 and 4, with very low activation energies for ring inversion (6.6 a nd 2.2 kcal mol (1), respectively), but ruled out solid state fluxionality for 4, whose observed planar geometry must be attributed to packing stabili zation.