Cobalt hydrido complexes

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

The first catalyst used in hydroformylation was cobalt. Under hydroformylation conditions at high pressure of carbon monoxide and hydrogen, a hydrido-cobalt-tetracarbonyl complex (HCo(CO)4) is formed from precursors like cobalt acetate (Fig. 4). This complex is commonly accepted as the catalytic active species in the cobalt-catalyzed hydroformylation entering the reaction cycle according to Heck and Breslow (1960) (Fig. 5) [20-23]. 

The hydrido-cobalt-tetracarbonyl complex (I) undergoes a CO-dissocia-tion reaction to form the 16-electron species HCo(CO)3 (II). This structure forms a 7r-complex (III) with the substrate and is a possible explanation for the formation of further (C = C)-double bond isomers of the substrate. In the... 

In the next step of the reaction cycle, the carbon monoxide is inserted into the carbon-cobalt bond. At this time, the subsequent aldehyde can be considered as preformed. This step leads to the 16 electron species (VI). Once again, carbon monoxide is associated to end up in the 18 electron species (VII). In the last step of the reaction cycle, hydrogen is added to release the catalyti-cally active hydrido-cobalt-tetracarbonyl complex (I). Likewise, the aldehyde is formed by a final reductive elimination step. 

Further reduction to cobalt (I) further increases the electron population of the coordination center and the radical-bonding properties of cobalt are no longer favored. Instead, the EPD properties that prevail at the coordination center allow coordination by EPA units according to the second stabilizing rule the complex ion is stabilized ) as a hydrido complex ... 

Griffith and Wilkinson, in a nuclear magnetic resonance study (3), found that a hydrido complex was formed in quantitative yield on treatment of cyano-cobaltate(II) solution with sodium borohydride. A hydrido complex was also present to the extent of 3% in a solution which had not been so treated. Furthermore, saturation of the solution with hydrogen, or aging, did not increase the amount of hydrido species, and it was suggested that these latter processes involved the formation of a nonhydridic cobalt(I) species. 

Since the aging reaction of cyanocobaltate(II) results in the formation of hydrido complex, the question arises as to which cobalt species is involved in the absorption of butadiene. If the hydride is the reactive species, absorption would be expected to increase with time. In Figure 3 it may be seen that the absorption of butadiene by cyanocobaltate(II) does increase with time in a manner paralleling the decrease in hydrogen absorption capacity (12). 

Reactions with Hydrido Complex. Upon injection of a prehydrogenated cyanocobaltate(II) solution (0.15M cobalt, CN/Co = 6.0) into an atmosphere of butadiene, the gas was rapidly absorbed, 0.92 mole of butadiene being taken up for each hydrogen atom previously absorbed. Similarly, when the injection was made into a butadiene-saturated cyanocobaltate(II) solution in a butadiene atmosphere, 1.08 moles of butadiene were absorbed. These results provide evidence of the addition of butadiene to the hydrido complex in the following manner ... 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydrido complex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)s could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

Paolo Chini began his work in the late 1950s with the characterization of cobalt carbonyl species involved in the hydroformylation of olefins with cobalt catalysts, and in the course of these studies developed improved synthetic methods for the known cobalt carbonyls Co2(CO)8 and Co4(CO)12 [132]. His next steps were the preparation of the heterometallic hydrido complex HFe-Co3(CO)i2 (isoelectronic to Co4(CO)12) and the corresponding anion [FeCo3(CO)12], both a novelty at that time, and of the new hexanuclear cobalt clusters [Co6(CO)15]2, [Co6(CO)14]4, and Co6(CO)16 [133-139]. This work was followed by the synthesis of carbido carbonyl cluster anions [Co6(CO)i4C], [Co6(CO)15C]2 and [Co8(CO)i8C]2, containing an interstitial... 

Acylphenolato(hydrido)cobalt(lll) complexes were found to react smoothly with 2-nitrophenol, according to Equation (4) <2003ICA179>. 

The high reactivity of this complex toward molecular hydrogen is basic to its function as a catalyst in the hydrogenation of various organic compounds 19, 20, 21). While some doubt remains concerning the detailed mechanism of this reaction (9,10,16), the net result is a reversible homolytic process in which hydrogen may be considered to be reduced to hydride, and cobalt(II) to be oxidized to cobalt(III) (Reaction 1). The reactive hydrido complex (the name used for Co(CN)-,H through-... 

Table II. Effect of Hydrido Complex Concentration on Products of the Reduction of Butadiene and Butenyl Chlorides at a Cyanide to Cobalt Ratio of 5.1...

Table II indicates the isomer distribution of the butene product obtained at a cyanide to cobalt ratio of 5.1 under two reaction conditions. The upper set of figures for each substrate refers to the product distribution obtained in the presence of excess hydrido complex. The lower set of figures refers to results obtained with pentacyanocobaltate(II) in an inert atmosphere. Under such conditions, hydrido complex is formed via cleavage of water (Reaction 2).

Such cobalt(I) complexes are extremely air-sensitive, low spin d systems, some of which are believed to be 4-coordinate square-planar and all of which are highly nucleophilic reagents due to the lone pair behavior of the d electrons. These nucleophilic cobalt(I) chelates are the conjugate bases of hydrido-cobalt complexes (Eqn. 1) [23,26] which in turn are unstable, decomposing to hydrogen and cobalt(II) (Eqn. 2) [23,27,28], a reaction which is known to be reversible, at least in some cases. 

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