Carbonyl species

A whole range of cobalt complexes has been observed under syn-gas pressure. The actual complexes formed will depend strongly on the conditions. 

The tetranuclear and trinuclear clusters will only be observed at low pressures [8], but all other species are very common under hydroformylation conditions. Complex 4 is an ionic complex that is formed in polar solvents [9] and even hexa-solvated, divalent cobalt species may form as the cation. Under practical conditions both the dimers and the hydrides are observed, thus depending on the hydrogen pressure there will be more or less of the hydride present. 

The reaction conditions chosen for the in situ IR studies are often much milder (25-100 °C, 100 bar) than the process conditions (140 °C, 250 bar), which may not affect the equilibria, as the higher pressure compensates for the higher temperature. The relative rate, however, of isomerisation of 1-alkenes may increase dramatically between room temperature and 140 °C. 

It is likely that the saturated complex Co2(CO)8 loses one molecule of CO to give unsaturated Co2(CO)7 before reacting with H2. In spite of the extremely fast exchange of free CO with coordinated CO the regeneration of HCo(CO)4 remains a slow process. 

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The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

The observed increase in CH formation and simultaneous decrease in CO formation with increasing catalyst potential and work function i.e. with increasing supply of O2 to the catalyst is remarkable and can be attributed to the preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH4 and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.59... 

Of particular interest is an electrochemistry study on related Mn(CNR) complexes, and on several carbonyl species 154). The one-electron oxidation process in acetonitrile (Mn(CNR)6+ Mn(CNR)s +) is substantially... 

In addition, hydrogen bonding was still observed at 3,583 cm (Fig.l A). IR peaks in VC.O region (Fig.l B) shifted to lower frequency after adsorption due to precursor-support interaction and differed from that of cluster solution implying that precursor was not intact after adsorption on support but still in the form of metal carbonyl species. 

Scheme 5 Schematic picture of CO addition to isolated Cr(II) species, according to the multiple CO addition model [48,53,54,77,99]. Carbonyl species observable at RT are shown in gray carbonyl species observable at 77 K are shown in black...

We have shown in the preceding section that the IR spectra of well-defined metal carbonyls provide valuable information of the environment of the deposited metal atom. However, IR signals of CO molecules adsorbed on larger particles suffer from broad lines, which hamper a more detailed analysis of the data. In the forthcoming section we will present results on cobalt particles where carbonyl species are formed on larger particles containing hundreds of atoms. 

The presence of a site with a low metal-metal coordination is compatible with the non-crystalline nature of the cobalt deposits [64]. It is to be expected that these sites exhibit different chemical reactivity than the usual adsorption sites. This can be verified by subsequent deposition of a small amount (0.1 A) of Pd atoms, which are known to nucleate exclusively on the cobalt particles [64]. The corresponding IR spectrum is shown as the bottom trace in Fig. 6. It is seen that an additional peak appears at 2105 cm which is readily assigned to CO bound terminally to Pd. More importantly, the growth of this Pd feature is completely at the expense of the carbonyl species, indicating that Pd nucleates almost exclusively at these low coordinated sites and prevents the formation of the carbonyl species. 

Aldini, G., Facino, R. M.. Beretta, G., Carini, M. Carnosine and related dipeptides as quenchers of reactive carbonyl species from structural studies to therapeutic perspectives. Eiofactors 2005, 24, 77-87. 

Beden B, Lamy C, Bewick A, Kunimatsu K. 1981. Electrosorption of methanol on a platinum electrode. IR spectroscopic evidence for adsorbed carbonyl species. J Electroanal Chem 121 343-347. 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

For a ToF-SIMS investigation of the surface oxidative degradation of low-density polyethylene (LDPE), the polymer was exposed to 1802 rather than 1602 in order to be able to readily discriminate oxygen introduced by the ageing process from that in the polymer prior to ageing [102], Figure 36 shows an example series of ToF-SIMS spectra from this investigation, which shows the clear separation of the lsO species from the lsO species. In the study, close correlation was observed between the intensity of the lsO carbonyl species determined by mid-infrared spectroscopy with the ToF-SIMS 180- peak intensity as a function of 1802 exposure time. ToF-SIMS spectra obtained from microtomed cross-sections showed no... 

These reactions had similar rate constants, 4 x 109 dm3 mol-1 second-1, which approached the diffusion-controlled limit. Thus, for 10-2 M concentration of added ligand the half-life of Cr(CO)5 would be 17 nseconds. Interest in these experiments has been reawakened by the recent reports of photoactivation of alkanes by metal carbonyl species 34). 

Flash photolysis has now been applied to a wide range of metal carbonyl species in solution, including Mn2(CO)10 (37), [CpFe(CO)2]2 (38), and [CpMo(CO)3]2 (39). In almost every case, interesting data have emerged, but, as with Cr(CO)5, the structural information is usually minimal. Thus, the radical Mn(CO)5 has been generated in solution by flash photolysis (37), the rate constant for its bimolecular recombination has been measured, but the experiments did not show whether it had Z>3h or Qv symmetry. Some experiments have been unsuccessful. Although the fragment Fe(CO)4 is well known in matrices (15), it has never been... 

The relative yields of the photoproducts Fe(CO)4, Fe(CO)3, and Fe(CO)2 were strongly wavelength dependent, and the relative amount of Fe(CO)2 increased as shorter wavelength uv light was used. The formation of all of these photoproducts appeared to involve absorption of only a single uv photon by a molecule of Fe(CO)5 (46,68,75). This is quite different from photochemistry in a matrix. When Fe(CO)3 and Fe(CO)2 are produced photolytically from matrix-isolated Fe(CO)5, one uv photon is required to remove each CO group, and the formation of the lower carbonyl species is a multistep process (87,88). 

Already a considerable number of transient organometallic species have been characterized by IR kinetic spectroscopy (see Table I). Like most other sporting techniques for structure determination, IR kinetic spectroscopy will not always provide a complete solution to every problem. What it can do is to provide more structural information, about metal carbonyl species at least, than conventional uv-visible flash photolysis. This structural information is obtained without loss of kinetic data, which can even be more precise than data from the corresponding uv-visible... 

Demerseman and co-workers have briefly noted that the reduction of CpTiCl3 in THF with either aluminum or magnesium in the presence of CO gave Cp2Ti(CO)2 as the only titanium carbonyl species (27). 

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