Reactions of Coordinated CO

Reaction of coordinated CO with less active nucleophiles can take place when the carbonyl ligand is sufficiently activated. The manner in which this activation occurs is by a reduction in the backbonding interaction (60) that may be achieved when the CO-bound metal ion is in a higher oxidation state. 

The discussion of typical reactions of coordinated CO should be seen as an introduction to the reactivity of other unsaturated molecules bound to, and activated by, transition metals. 

Of greater potential interest is the reaction of coordinated CO with with reactants that attack at... 

It should be pointed out, however, that although many of the dioxygen complexes react readily with SO2 to give sulfato complexes, many of the analogous SO2 complexes react more slowly, or not at all with molecular oxygen. Reactions of coordinated CO also give rise to carbonate complexes except in the case of rhodium complexes which tend to form coordinated or free CO2, equations (47)-(50). 

In organometallic chemistry, stable methylene-bridged bimetallic complexes and methyne-capped trimetallic clusters are common, and rare carbide complexes have occasionally been formed by reaction of coordinated CO in clusters (for instance, Hubei s nido cluster shown below) ... 

The 8-coordinate species [Mo(Et2dtc)4] can be obtained by reaction of Mo(CO)e with tetraethylthiuram disulfide (1 2) in acetone under N2. The X-ray structure revealed square-antiprismatic coordination, with a crystallographic, twofold axis coinciding with the molecular pseudo 4 axis (158). The magnetism and spectra of [M(dtc)4]"" (M = Mo or W n = 0 or 1) have been interpreted in terms of dodecahedral symmetry (159). 

Kinetic studies have been made of the reaction of CpMo(CO)j R (R = Me, Et, CH2Ph, and CH2CH=CH2) (48, 80, 81) and 7r-X2C9H5Mo(CO)jMe (X = H or OMe) (108) with a variety of P donor ligands L. Solvents employed ranged from nonpolar hexane to polar THF and MeCN. Generally, the mechanism is very sensitive to the coordinating ability of the solvent and the nucleophilicity of L. 

As already pointed out, the presence of a macrocyclic ligand, which occupies the four equatorial coordination sites of an octahedral complex, will tend to limit insertion and elimination reactions of the Co—C bond to those which require only that single coordination site. The presence of unidentate ligands in the pentacyanides, on the other hand, will offer greater opportunities for reactions which require a second, adjacent site. 

Mononuclear acyl Co carbonyl complexes ROC(0)Co(CO)4 result from reaction of Co2(CO)8 with RO-.77 These also form via the carbonylation of the alkyl precursor. The ROC(0)Co(CO)4 species undergo a range of reactions, including CO ligand substitution (by phosphines, for example), decarbonylation to the alkyl species, isomerization, and reactions of the coordinated acyl group involving either nucleophilic attack at the C or electrophilic attack at the O atom. 

A rare example of thiourea coordination to low-valent Co is of a disubstituted thiourea as bridging ligand, observed in the cluster Co3(CO)7(/i3-S)(/i- 72-PhNC(S)NHCH2Ph) which is formed by reaction of Co2(CO)8 with the thiourea.172 The crystal structure of the product defines a tetrahedral Co3S core with all carbonyls in terminal positions and the deprotonated thiourea bridging two Co centers via the S and an amido N. 

Organic carbamates (RNHCOO-) commonly display monodentate coordination, as exemplified in the structurally characterized tetrahedral Co(bmc)2Cl2,438 (bmc = lV-(benzimidazoyl-2-yl)-O-methylcarbamate). An unusual route to a carbamato complex involves the reaction of Co2(CO)8 in the presence of a fourfold excess of the stable radical species tmpo, which yields the blue Co40(OOCNC9H18)6 cluster, presumably via a Co(CO)2(tmpo) intermediate, with the nitroxyl radical serving as oxidant.439... 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

This reaction takes place because of the ease of formation of the carbonylate ions and the favorable coordination of the Fe2+ produced. The reaction of Co2(CO)8 with NH3 is similar. 

CH3Mn(CO)4THF has been identified, so the conjecture that THF and DMF behave in a manner that is different from the other solvents is not without some justification. Additional work on the influence of the solvent could yield additional insight regarding the mechanisms of this and other reactions of coordinated ligands. 

Organochromium compounds with various coordination modes of the ligand, namely CpCr(CO)2S2P(OPr )2 (bidentate), CpCr[S2P(OPr )2]2 (one bident-ate and one unidentate) and Cr[S2P(OlV)2]3 (bidentate) are formed in reactions of [CpCr(CO)3]2 with (Pr 0)2(S)PSSP(S)(0Pr )2. Variable temperature NMR shows rapid interchange between bi- and unidentate ligands in solutions of CpCr[S2P(OPr )2]2.61... 

The attack on coordinated carbon monoxide by nucleophiles was first extensively developed in synthetic organometallic chemistry by E. 0. Fischer and his students (6) as discussed by others in this volume, this reaction provides one route to the reduction of coordinated CO and to catalysis of the water gas shift reaction. Those carbonyl groups which are susceptible to attack by nucleophiles are electron deficient, as judged by their high CO stretcing frequencies (7). 

The above observations strongly indicate that O-protonation is an important step in this particular reaction for the reduction of coordinated CO. Recent studies in our laboratory provide other examples of proton induced reduction in metal cluster systems, and an example of proton induced CO reduction has recently been reported by Atwood (44). It thus appears that protons as well as Lewis acids are effective in the bifunctional activation of coordinated CO. 

The foregoing examples clearly demonstrate that the attachment of an electron acceptor to the oxygen of coordinated CO activates this molecule toward a variety of reduction reactions. 

We would like at this time to amplify the earlier brief comments on the catalytic decomposition of formate ion to H2 and C02 by the group 6b metal carbonyls. This process requires the presence of a vacant coordination site on the metal for formate binding, i.e., formation of m(C0)s02CH species. Consequently, the reaction of Cr(Co)0 with formate ion in 2-ethoxyetha-nol was found to take place under more rigorous conditions than those needed for the production of H2 in the Cr(C0)e/K0H system. That is, whereas Cr(C07e in aqueous K0H/2-ethoxyethanol commences to produce H2 at 50°C (where the rds appears to succeed the formation of Cr(C0)sH ), the analogous Cr(CO)e/formate ion reaction necessitates temperatures approaching 100°C. 

New NMR information on the state of coordinated 2-formylglycinate in [Co(en)2(formgly)], showing it to be in hydrate plus enol form rather than in aldehyde form, has led to a new theory for the mechanism of the reaction of coordinated formylglycinate with penicillamine (402). 

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