Metal carbonyls, mononuclear reactions

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

The rates of the water gas shift reactions were compared using different amounts of the mononuclear metal carbonyl precursor for all four cases (Fe(CO) and M(CO) where M = Cr, Mo, and W). In all cases the rates of hydrogen production were found to double as the concentration of the metal carbonyl was doubled. Thus all of the water gas shift reactions investigated... 

Many of the methods used for the preparation of mononuclear hydrides may be applied to the polynuclear systems. Base attack on metal carbonyls, which furnished one of the first methods for the production of carbonyl hydride species, is applicable to a wide range of carbonyls. Borohydride reduction leads to a variety of products, depending upon the reaction conditions, Os3(CO)12 reacting with NaBH in di-oxane under reflux to give, after 4 hours, a mixture of the anions [H30s4(C0)12] and [H2Os4(CO)i2]2 (79). The related reaction in tetra-hydrofuran for 1 hour yields the anion [HOs3(CO)n]- as the main product with minor amounts of the two tetranuclear anions. 

The metal carbonyl clusters correspond to situations intermediate between metals and simple mononuclear or binuclear carbonyls. Their existence must be connected either with a delicate thermodynamic balance or with remarkably high activation energies. The last hypothesis is valid for species such as Rh6(CO)j6 which is kineti-cally inert, but in general we are inclined to believe that thermodynamic control is the more significant, especially since reactions, such as that shown in Eq. (1), can be carried out in both directions using mild conditions. 

Oligo- and polymethylene-bridged complexes can be prepared by reaction of metal carbonyl anions with w.w -dihaloalkanes. This method proved to be a simple entry to the iron series /j,-(CH2)j.][(t75-C5H5) Fe(CO)2]2 (x s 3) (283), but yielded quite different products in the case of Na[(T 5-C5H5)Mo(CO)3] (283) and Na[Mn(CO)s] (284). An alternative two-step synthesis of the iron compounds involves the preparation of the mononuclear w-haloalkyls LXM—(CH2)X—X, and a subsequent... 

Exclusive formation of silylstyrenes 76 is achieved when the reactions of styrene and 4-substituted styrenes with HSiEt3 are catalyzed by Fe3(CO)i2 or Fe2(CO)9100. Other iron-triad metal carbonyl clusters, Ru3(CO)i2 and Os3(CO)i2, are also highly active catalysts, but a trace amount of hydrosilylation product 77 is detected in the Ru-catalyzed reactions and the Os-catalyzed reactions are accompanied by 3-12% of 77 (equation 31)100. Mononuclear iron carbonyl, Fe(CO)5, is found to be inactive in this reaction100. 

Hydrosilylation reactions catalyzed by iron carbonyl compounds often occur under drastic thermal conditions. Schroeder and Wrighton reported a photocatalyzed reaction oftrialkylsilanes with alkenes in the presence of Fe(CO)5 at low temperatures (0-50 °C) [48]. It is well known that irradiation of mononuclear metal carbonyls leads... 

Decatrienes, in intramolecular Alder-ene reactions, 10, 578 Dechlorination reactions, via cobalt(III) complexes, 7, 44 Decomposition pathways mixed metal carbonyls, via sonochemistry, 1, 310 in mononuclear ruthenium and osmium alkenyl formations, 6, 405... 

In both the examples given above, there is concomitant loss of one or more neutral ligands. Elimination of CO is the rule in reactions of mononuclear metal carbonyls (e.g., entry 12) and cyclopentadienyl metal carbonyls (e.g., entry 4), but not those of polynuclear carbonyls (e.g., entry 16) or carbonyl halides (e.g., entry 33). Elimination of tertiary phosphines often occurs, especially when more than two molecules are present in the initial complex however, this is not always the case (see entry 24). Clearly, steric requirements and the dictates of the 18-electron rule determine the composition of the product, and normally act in concert when they conflict, as in the case of R3SiRuH3(PR3) (n = 2 or 3 entry 22), variable stoichiometry may result. Chelating diphosphines, with somewhat reduced steric requirements, are usually retained (e.g., entry 19), while complexed olefins are invariably lost the bulky ligand P(cyclohexyl)3 is associated with unusual products (entries 47 and 48). Particular mention may be made of the 17-electron species Cl3SiVH(Cp)2 and (Cl3Si)2V(Cp)2 shown... 

For such experiments, however, liquid ammonia is not suitable. A much more promising reaction offered by that discovered by Wannagat and Seyffert (149) involves treatment of sodium bis(trimethylsilyl)amide with mononuclear metal carbonyls in which nucleophilic attack of the silyl amide on a CO ligand leads to monocyanocarbonyl metalates. 

With mononuclear transition metal carbonyls, photochemically induced substitutions of one or two carbonyl ligands by a diene can be expected to be the only reactions. Dinuclear transition metal carbonyls or carbonyl... 

Molecularly or ionically dispersed metal carbonyl clusters on metal oxides have been prepared in high yields by reaction of metal carbonyl clusters with support surfaces or by syntheses on support surfaces from mononuclear precursors (Gates and Lamb, 1989 Iwasawa, 1993 Ichikawa, 1992 Gates, 1994). Synthesis of supported metal carbonyl clusters has been reviewed recently (Gates, 1995,1998), and only a few examples are included here. 

For mononuclear complexes, reactions at coordinated ligands is a topic of major study, and numerous reviews and even books have been written on this topic. At the time of writing this review, the authors could find no major review articles in this area for dinuclear compoimds. The best examples are probably those on the reactions of coordinated molybdenum sulfides discussed earlier. A related observation is the reaction of metal carbonyls with Lewis acids. It has been observed by Shriver and coworkers that carbonyl ligands bridging metal-metal bond atoms show increased tendency to coordinate Lewis acids (equation 96). 

Few successful reactions are known by which an organometallic nitrosyl product has been formed from the condensation of mononuclear metal nitrosyl complexes. The unique cluster (Tj -C5H5)3Mn3(NO)4 (39), which contains the only i3-NO known (7) (40), is prepared using photochemical or thermal (41) decarbonylation of (Tj -C5H5)2Mn2(CO)2(NO)2 [Eq. (25)]. The use of the redox condensation of a nitrosyl carbonylmetallate with a neutral metal carbonyl cluster to produce a nitrosyl cluster has... 

S.3.2.3. by Reaction with a Mononuclear Transition- or Inner Transition-Metal Carbonyl in the Presence of Ultraviolet Radiation. 

It is probable that the course of reactions discussed here is photochemical CO ejection followed by oxidative addition of Si—H to the 16-electron metal center this is reflected in the nature of the products (Table 1). Few reactions of mononuclear metal carbonyls with silanes are induced thermally in preference, and these often lead to different products. For example, M(CO)2C5H5-t (M = Co, Rh) and Cl3SiH give M(SiCl3)2(CO)CjH5t at 100"C and RT respectively whereas FefCO), and the same silane provide Fe(SiCl3)2(CO)4 at 140°C and Fe3(CO),2(/<-SiCl2)3 at 160°C . 

S.8.4.3.2. by Reaction of Germanium Tetrahalldes with Neutral Mononuclear Metal Carbonyls and Related Species. 

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