SYNTHESES, X-RAY CRYSTAL-STRUCTURES, AND FLUXIONAL BEHAVIOR OF THE CLUSTERS CP-ASTERISK-CO-3(3)(MU(2)-CO)(MU(3)-CO)(MU-H)(2), STERISK-CO-3(3)(MU(2)-H)(MU(3)-ETA(2)-HC=NCME(3)), AND RISK-3CO3(MU(2)-H)(MU(3)-ETA(2)-HC=NCME(2)CH(2)ME)

CpCo-3(3)(mu(2)-H)(3)(mu(3)-H) (1) reacts with 2 equiv of CO to form the 48-electron dicarbonyl dihydride cluster CpCo-3(3)(mu(2)-CO)(mu(3 )-CO)(mu-H)(2) (2), which was studied by X-ray crystallography. 2 cons ists of an equilateral triangle of cobalt atoms (Co-Co = 2.476(1) Angs trom) capped on each face by a mu-CO ligand. Line-shape analysis of th e CO region in the variable temperature C-13 NMR Of 2 allowed the barr ier for interconversion of the mu(2)- and mu(3)-CO ligands to be measu red (Delta G(double dagger) = 8.6(1) kcal mol(-1)). Cluster 2 loses H- 2 at 80 degrees C (Delta G(double dagger) = 26(1) kcal mol(-1)) to for m the 46-electron dicarbonyl cluster CpCo-3(3)(mu(3)-CO)(2) (3). 3 re acts with 5.4 atm of H-2 at 80 degrees C to form a 1.1:1 equilibrium m ixture of 2 and 3 (Delta G = -2.6(1) kcal mol(-1)). tert-Butyl isocyan ide reacts with 1 at -35 degrees C to form the bis(isocyanide) cluster CpCo-3(3)(mu-CNCMe(3))(2)(mu-H)(2) (10), which was observed by low-t emperature H-1 NMR spectroscopy. A coordinated tert-butyl isocyanide l igand rapidly inserted into a cobalt-hydrogen bond of 10 to produce th e formimidoyl cluster CpCo-3(3)(mu 2-H)(mu(3)-eta(2)-CH=NCMe(3)) (4). Likewise, the reaction of tert-amyl isocyanide with 1 formed pCo-3(3 )(mu(2)-H)(mu(3)-eta(2)-CH=NCMe(2)CH(2)Me) (11). Clusters 4 and 11 wer e characterized by X-ray crystallography and were shown to consist of a triangle of cobalt atoms capped on one face by a mu(3)-eta(2)-formim idoyl ligand and capped on the opposite face by a mu-hydride ligand. L ine shape analysis of the Cp region in the variable temperature H-1 N MR of cluster 4 allowed the barrier for complete rotation of the formi midoyl ligand parallel to the tricobalt plane to be measured (Delta G( double dagger) = 9.9(1) kcal mol(-1)).