Selectivity in substitution chemistry of mixed-metal, tetrahedral MCo3 (M = iron, ruthenium) carbonyl clusters: cobalt-59 NMR study and crystal structure of RuCo3(.mu.3-H)(.mu.-CO)3(CO)8(NMe3)

Reactions between tetrahedral mixed-metal clusters HMCosfCO^ [M = Fe (la), M = Ru (lb)] and trimethylamine IV-oxide have led to the formation of the amine-substituted clusters HMCo3(CO)11(NMe3) [M = Fe (2a), M = Ru (2b)] in high yields. 69Co NMR and IR spectroscopy were used to study the site selectivity of these reactions and indicate that substitution of the amine for the CO ligand takes place preferentially at one of the cobalt atoms. Clusters 2 are labile in solution and transform in the corresponding anions [Me3NH] [MCoslCO)^], owing to decoordination of the amine. Substitution reactions of the amine with other 2e donor ligands (PPh3, SEt^) were also studied. Disubstituted clusters were prepared under mild conditions and the very labile HMCo3(CO)i0(NMe3)2, in which two cobalt atoms carry each an amine ligand, was characterized by 59Co NMR spectroscopy. The structure of 2b has been determined by X-ray diffraction: space with a = 16.877 (5) A,b = 10.296 (2) A, c = 11.946 (2) A, a - ß = y = 90°, and Z = 4. The structure was refined to R = 0.026 and R„ = 0.035 on the basis of 1471 reflections having F1 2 > 3 (F02). The cluster has a tetrahedral structure and the amine is axially bonded to a basal cobalt atom. The problem of ligand exchange or loss in the formation of organometallic alkaline-earth-transition-metal complexes can be minimized by ensuring that any exchange would be degenerate or would require an unfavorable transfer of ligands. This approach is illustrated in the formation of chloride-bridged calci-um-group 4 dimers and in an isocarbonyl-linked calcium-chromium dimer. Bis(pentamethylcyclo-pentadienyl)calcium Cp*2Ca, reacts with Cp*2ZrCl2, Cp*2HfCl2, and Cp*2ThCl2 in toluene to yield 1:1 adducts. Orange-yellow crystals of Cp*2HfCl(/i-Cl)CaCp*2 grown from toluene are triclinic, space group PI, with a = 11.146 (4) A, b = 16.942 (6) A, c = 11.109 (3) A, a - 103.34 (3)°, ß = 94.89 (3)e, 7 = 102.50 (3)°, and D(calcd) = 1.398 g cm"3 for Z = 2. The structure consists of two bent metallocene units with a single µ-Cl and a terminal Cl on hafnium: ß-µ-= 2.864 (3) A, -µ-Cl = 2.463