Higher-nuclearity Carbonyls

Three routes have been identified from the kinetics where the phosphorus ligand is PBua, PPhs, or P(OPh)3. A rate-determining loss of L is suppressed with sufficient of the ligand present, but rate-determining loss of CO and a process involving rate-determining fragmentation independent of [CO] or [L] operate. The CO dissociation path is believed to be 

The natures of the two octacarbonyl species have been discussed. 

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The carbonyl platinum anions, [Pt3(CO)6]2, (n = 1-6,10) were first synthesized and characterized by Chini and coworkers1 3. They obtained these compounds by reaction of Pt(IV) or Pt(II) salts at room temperature with bases such as sodium hydroxide or sodium acetate under a carbon monoxide atmosphere. The product composition is quite sensitive to the Pt-base ratio, reaction time, and reaction conditions. As a consequence of this sensitivity, product mixtures with An = 1 are usually obtained, which are separable only with difficulty by fractional crystallization. Interest in this series of compounds for (a) their unique redox solution chemistry, (b) their use as precursors for higher nuclearity carbonyl platinum anions,4 and (c) their use as precursors for novel supported Pt catalysts5 8 prompted efforts to develop... 

Phosphorus very readily forms complexes with iron carbonyl fragments to give mono- and disubstituted Fe(CO)4L and Fe(CO)3L2 complexes. These are common derivatives to prepare and may be made directly from Fe(CO)s or from the higher nuclearity carbonyls Fe2(CO)9 or Fc3(CO)i2. Dinuclear Fc2(CO)9 is not known to be soluble in any common organic solvents with retention of its composition but it does dissolve in coordinating solvents, THF being... 

Reduction of the pH of solutions of carbonylate anions yields a variety of protonated species and, from acid solutions, carbonyl hydrides such as the unstable, gaseous H2Fe(CO)4 and the polymeric liquids H2Fe2(CO)g and H2Fe3(CO)n are liberated. The use of ligand-replacement reactions to yield hydrides of higher nuclearity has already been noted. 

Addition of either nucleophilic or electrophilic metallic species can result in the capping of triangular- or square-metal faces in carbonyl clusters. These redox reactions provide high yield syntheses of higher nuclearity clusters and somewhat resemble surface reconstruction on metals. With a few examples,... 

Platinum carbonylate anion clusters like [Pt3(CO)6] can be obtained by alkaline reduction of [PtCh] in a CO atmosphere. From [Pt3(CO)s] other higher nuclear-ity anions can be obtained. In this context, several examples have been reported in which this type of anionic cluster is used in the preparation of catalysts by impregnation or exchange methods. Salts of [Pt3 (CO)6 ] (n = 3, 5) have been used to prepare, by impregnation, dispersed platinum on ZnO and MgO [49] and, by ion exchange methods, to prepare Pt3 /C electrodes for the electrocatalytic oxidation of methanol [50]. A salt of [Pti2(CO)24] has recently been used to prepare... 

Metal carbonyl anions react with main group halides and oxides to yield a number of main-group transition-metal carbonyl complexes in good yields. These complexes serve as starting materials for a number of higher nuclearity cluster complexes. 

Tetraethylammoniumhis(tetracarbonyliron)bis(/i-tetracarbonyliron)dithal-late(2 — ), [Et4N]2[Tl2Fe4(CO)16], decomposes rapidly upon exposure to air and is soluble in MeOH, CH2C12, and most polar organic solvents. Its crystal structure shows it to be a weak dimer of [Tl Fe(CO)4 2] and it probably exists as the monomer in solution.15 IR (CH2C12, cm-1) 1985(m), and 1908(s). It may be oxidized or irradiated to yield higher nuclearity thallium-iron carbonyl compounds.14,15... 

Two potential problems (opportunities ) which may be encountered are (i) redox condensation, whereby carbonylates of higher nuclearity may be formed (Figure 3.5a,d,e) effectively, the reduced carbonylate replaces a carbonyl ligand on combination with a second equivalent of the original neutral carbonyl (ii) over-reduction, whereby more than two electrons may be added, e.g. in the formation of Na4[Cr(CO)J or Na3[Mn(CO)4] (Figure 3.5b). These problems are controlled by judicious (or empirical) choice of solvent and reductant. 

Both its solubility and any potential side reaction with reductant dictate the choice of precursor metal salt or complex. In many cases the reductant is an aluminium alkyl or electropositive metal (Goups 1,2, 12 or 13, possibly amalgamated with mercury), requiring the use of anhydrous metal salts or complexes. Carbonyls of higher nuclearity are... 

Although anionic species with nickel in the oxidation state -I have been postulated, such as [Ni2(CO)6]2, these have not been confirmed. Nickel does, however, form a number of anionic carbonyl clusters with higher nuclearity, e.g., [Ni5(CO)l2]2-, in which the metal carries a partial negative charge. 

Low-valent palladium and platinum form numerous clusters, usually based on the M3 triangle, as in the carbonyl phosphine complexes 18-H-I. Higher nuclearity clusters can be built up from edge-sharing triangles, e.g., the butterfly structure 18-H-II. 

From the other directions, there have been a number of studies of the cluster species that are formed when mononuclear iridium precursors are deposited onto inorganic supports and then subjected to carbon monoxide pressures. Gates and coworkers have shown that Ir(CO)2(acac) will form higher nuclearity iridium carbonyl clusters, the exact nature of which depends on the substrate and the carbonylation conditions. For zeolite NaY, they have observed that Ir(CO)2(acac) will yield both It4(CO)i2 (45) and fr6(CO)i6... 

Carbonylation of different palladium complexes, often in the presence of phosphines, led to the synthesis of a variety of clusters including halocarbonyl clusters of unknown nucle-arity [PdX(CO)] (X = Cl, Br), and phosphine-carbonyl clusters from tetrameric [Pd4(CO)s(PR3)4] to high nuclear-ity compounds such as [Pd38(/r -CO)4(/u. -CO)24(PEt3)i2]. From these preformed clusters, nanosized cluster compounds of higher nuclearity such as [Pd69(CO)36(PEt3)ig] can be prepared. ... 

The most stable carbonyl for ruthenium is the trimeric tetracarbonyl Ru3(CO)i2 (1), with Ru(CO)5 (2) and Ru2(CO)9 (3) being much less stable. Compounds (2) and (3) are prepared from Ru3(CO)i2 and revert to it on standing or irradiation. No neutral homoleptic see Homoleptic Compound) mthenium carbonyl of higher nuclearity, in particular no analog of Os6(CO)i8, is known. 

Known carbonyl hydrides of mthenium include the unstable HRu(CO)4, as well as the trinuclear H2Ru3(CO)n, tetranuclear H2Ru4(CO)i3 and H4Ru4(CO)i2, and complexes of even higher nuclearity, as well as substitution and deprotonation derivatives. A special feature of mthenium carbonyl chemistry is the existence of series of carbonyl... 

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