Hydrido cobalt

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. 

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

Char formation and reduced monomer production are observed for all of these additives upon reaction with PMMA. Char formation increases as a function of temperature, for the hydrido cobalt compound, there is 5% char at 262°, 8.5% at 322°, 15% at 338°, and 19% at 375°C the cobalt(lll) cyanide produces 3% char at 338° and 11% at 375°C the cobalt(ll) cyanide yields 11% char at 375°C. At the highest temperature, 375°C, the amount of monomer formation is 22% for K3Co(CN)5, 11% for K3Co(CN)6, and 10% for HCo[P(OPh)3]4. Ideally one would hope to observe no monomer formation and complete char production. Such is not the case here, these materials probably have no utility as flame retardant additives for PMMA since monomer formation, even at a reduced level, will still permit a propagation of the burning process. While somewhat positive results for these three additives do not prove the validity of the hypothesis, we take this to be a starting point in our search for suitable additives, further work is underway to refine the hypothesis and to identify other potential hydrogenation catalysts and other additives that may prove useful as flame retardants for PMMA... 

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 ... 

Upon optimization for this particular PKR, propargyl vinylsilyl ether 59 always afforded monocyclic products 60 after loss of silicon atom (Equation (31)). The yield was moderate, and it was possibly due to the formation of the presumed hydrido-cobalt species again. However, Pagenkopf optimized this transformation intensively to find that a small portion of water substantially improved the yield. ... 

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 ... 

PtCo207P2C H24, Cobalt, heptacar-bonyl[ 1,2-ethanediylbis(diphenyl-phosphine)]platinumdi-, 26 370 PtF,04P2SC,4H,s, Platinum(II), hydrido-... 

The decay involves attack by HO at phosphorus followed by hydride shift to cobalt or a direct hydride shift to cobalt from coordinated ammonia. The hydrido-cobalt intermediate so generated is believed to rapidly reduce another molecule of the hypophosphito complex in a post-rate-determining step to produce one equivalent of hypophosphite. 

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]. 

Several effects are related to the nature of the silicon halide. Hydrido halides usually react readily with a range of anions, while organosilicon halides may not for instance, HgSiCU reacts with Co(CO)J in ether giving good yields of silicon-cobalt derivatives (2), while MesSiCl does not (336). This is probably related in the main to the electron-accepting... 

---