Transition metal sulfido clusters

This section is concerned with transition metal clusters that, in addition to metal atoms, contain sulfur atoms in the cluster core (rather than just in the peripheral ligands). Metal-metal bonds often supplement sulfur-atom bridges in stabilizing the structures encountered. Generally, such clusters are likely to resemble the HDS active phases of heterogeneous metal-sulfide catalysts to some extent, e.g. in terms of coordination sphere and metallic oxidation states. Because of the large number of molecular metal-sulfide clusters now known, we shall focus on homometallic clusters of Mo(W) and heterobimetallic clusters of Mo(W)-Co(Ni) (next two sections), i.e. molecular clusters containing the elements that are relevant for industrial 

M0S2 and WS2 phases unpromoted by cobalt or nickel have HDS activity similar to that of CoMoS, NiMoS, and NiWS phases but at a much lower level. Several homometallic molecular cluster cores existj e.g. the cubane [M04S4] (59) (n = 4,5 or 6), the incomplete cubane [M 3S4] (M = Mo(60a),W(60b)), the spiro dicubane [MovSs] and the partly disulfide-bridged incomplete cubane [M03S(S2)3] +. 

Although all these homometallic molecular clusters have metal oxidation states in the interval +3 to -I-4, i.e. very close to those of heterogeneous catalysts, their current chemistry is often not directly relevant for HDS research. A few examples to the contrary are provided below. 

This section is concerned with molecular clusters that contain the same elements as the active CoMoS/NiMoS/NiWS phases in heterogeneous HDS catalysts. One 

Reaction of [(Cp)2Mo2( -S)2(yU-SH)2] in solution with [Fc2(CO)9], [Co2(CO)g] or [Ni(CO)4] gives ) the tetranuclear bimetallic clusters [Cp2Mo2Fe2(CO)6(//3-S)2(/ -8)2] (63), [Cp2Mo2Co2(CO)4(/ 3-S)2(//4-S)] (64) and [Cp2Mo2Ni2(CO)2(/i3-S)4] (65), respectively. Whereas the iron-molybdenum and nickel-molybdenum clusters are electron-precise, the cobalt-molybdenum cluster is electron deficient and contains the unusual ligand. 

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The coordination chemistry of the sulfido ligand is very varied. Scheme 4 illustrates the modes of coordination of to transition metal ions. The monosulfido ligand can act as a two-electron, terminal or /u-2-bridging donor, or up to a six-electron donor if it binds as /u.4-bridging or interstitial donor. It is, therefore, not surprising that a wide variety of binuclear and polynuclear sulfido clusters complexes can be prepared. 

Thiolato and S-based ligands have often been also used to bridge two metals with the formation of heterobimetallic complexes. The synthesis and reactivity of cubane-type sulfido clusters containing titanium-late transition metals have been reviewed.1776,1777... 

H2S, SO2, S and CS2 have all been used as sources of sulfido atoms to introduce atomic sulfur into transition metal clusters. For example, the reaction of a solution of [Rh(CO)2(acac)] and alkali carboxylates with H2S or SO2 at high tern-... 

B7.29 Designed syntheses for some transition metal clusters involving bridging-sulfido ligand... 

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