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The first trinuclear clusters containing NHC ligands are described; the compound [Ru3(Me2Im)(CO)11](Me2Im=1,3-dimethylimidazol-2-ylidene) is easily converted into [Ru3(mu-H)2(mu3-MeImCH)(CO)9] by a process involving the activation of two C-H bonds of a methyl group that is an example of degradation of a metal-coordinated NHC ligand under mild conditions.

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A PGeP pincer-type germylene, Ge(NCHPBu)CH, which contains two phosphane groups hanging from the N atoms of an N-heterocyclic germylene fragment, has been isolated for the first time. This compound has already furnished a rich transition metal derivative chemistry (Co, Rh, and Pd) that includes complexes containing bridging P,Ge,P-, chelating P,Ge- and pincer P,Ge,P-ligands.

The dithiolate-bridged complexes [M2(μ-bdt)(CO)6] (M = Fe (1), Ru (2), Os (3); bdt = benzene-1,2-dithiolate) have been prepared. X-ray diffraction studies have revealed that although the three compounds have analogous molecular structures their crystal structures are different as a result of different molecular packings. Complex 1 does not react with tetrafluoroboric acid in 1,2-dichloroethane, while compounds 2 and 3, under the same conditions, undergo protonation at the metal atoms to give the cationic hydrido derivatives [M2(μ-H)(μ-bdt)(CO)6][BF4] (M = Ru, Os). EHMO calculations have been used to rationalize the results of the protonation reactions.

The room-temperature reactions of [Ru 3 (CO) 12 ] and [Os 3 (CO) 12 ] with a variety of N-heterocyclic carbenes (NHCs) have been studied. [Ru 3 (CO) 12 ] reacts easily with N,N′-dimethylimidazol-2-ylidene (Me 2 Im), more slowly with N-methyloxazol-2-ylidene (MeOx), and very slowly with N,N′-dimesitylimidazol-2-ylidene (Mes 2 Im) to give the corresponding CO substitution products [Ru 3 (NHC)(CO) 11 ], but it does not react with the very bulky N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Dipph 2 Im). DFT calculations have shown that [Ru 3 (Dipph 2 Im)(CO) 11 ] is a minimum in the corresponding potential energy surface; therefore, the absence of reaction between [Ru 3 (CO) 12 ] and Dipph 2 Im has a kinetic origin associated with the large volume of this NHC. [Os 3 (CO) 12 ] reacts with Me 2 Im to give [Os 3 (Me 2 Im)(CO) 11 ]. However, MeOx is not basic enough and Mes 2 Im and Dipph 2 Im are too bulky to react with [Os 3 (CO) 12 ], which is less reactive than [Ru 3 (CO) 12 ]. Therefore, the reactions of [Ru 3 (CO) 12 ] and [Os 3 (CO) 12 ] with NHCs are strongly influenced by the electronic properties and steric demands of the NHCs and also by the intrinsic reactivity of the metal-carbonyls.

The cationic cluster complexes [Ru3(CO)10(mu-H)(mu-kappa2N,C-L1Me)]+ (3+; HL1=quinoxaline) and [Ru3(CO)10(mu-H)(mu-kappa2N,C-L2Me)]+ (5+; HL2=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru3(CO)10(mu-H)(mu-kappa2N,C-Ln)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N-heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris-sec-butylborohydride (K-selectride) to give neutral products that contain novel nonaromatic N-heterocyclic ligands. The following are the products that have been isolated: [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L1Me2)] (6; from 3+ and methyl lithium), [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L1HMe)] (7; from 3+ and K-selectride), [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L2Me2)] (8; from 5+ and methyl lithium), and [Ru3(CO)9(mu-H)(mu3-kappa2N,C-L2HMe)] (11; from 5+ and K-selectride). Whereas the reactions of 3+ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N-methyl group, the reactions of 5+ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N-methyl group. The LUMOs and the atomic charges of 3+ and 5+ confirm that the reactions of these clusters with anionic nucleophiles are orbital-controlled rather than charge-controlled processes. The N-heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face-capping modes. Those of compounds 6-8 have the atoms of a ligand C=N fragment sigma-bonded to two Ru atoms and pi-bonded to the other Ru atom, whereas the ligand of compound 11 has a C-N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable-temperature 1H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above -93 degrees C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH(2) group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180 degrees rotation of the entire organic ligand over a face of the metal triangle.

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