Unsaturated fragment

A similar procedure allows us to assign the additional peaks produced on ArF photolysis as being due to Cr(C0>3 and Cr(C0>2 [9]. The frequencies of the gas phase absorptions for these coordinatively unsaturated fragments is presented in table II. Data for the rates of reaction of each of these species with CO is summarized in table I. 

On the assumption that this suggestion is correct, cluster formation may then be considered to occur by combination of the unsaturated fragments such as Fe2(CO)7 or Os2(CO)7. Thus, the polymerization of... 

Insertion of carbon monoxide and alkenes into metal-carbon bonds is one of the most important reaction steps in homogeneous catalysis. It has been found for insertion processes of platinum [16] that the relative positions of the hydrocarbyl group and the unsaturated fragment must be cis in the reacting complex [17], The second issue concerns the stereochemical course of the reaction, insertion versus migration as discussed in Chapter 2.2. 

P-H activation by the unsaturated fragments directly bonded to the phosphorus... 

In general, there is no strong evidence to support homoaromatic formulations of the structures of any of these systems. There are indications from PE spectroscopy of some degree of interaction between the unsaturated fragments of these molecules. However, as we have pointed out, PE spectroscopy as a technique has limited value in probing homoaromaticity. Magnetic evidence has either not been examined in detail in most systems or, where chemical shifts have been examined, is not definitive. Thermochemical... 

Laboratory in Oxford, and Geoffrey Ozin at the University of Toronto in the early 1970s. With the metal atom cocondensation technique (which as described in Chaps. 6 and 7 was also used to prepare a series of zerovalent arene and olefin metal complexes), they reported simultaneously that the elusive palladium and platinum tetracarbonyls, Pd(CO)4 and Pt(CO)4, as well as the coordinatively unsaturated fragments M(CO)3, M(CO)2, and M(CO) (M = Pd, Pt) were formed by cocondensation reactions of Pd and Pt atoms with CO in inert gas matrices at 4-10 K [119-122]. The comparison of the CO bond stretching force constants for Pd(CO)ra and Pt(CO)ra (n - 1-4) revealed that, in analogy to Ni(CO) , the most stable compounds were the tetracarbonyls. In a xenon matrix, Pd(CO)4 existed up to about 80 K [120]. Ozin s group as well as others... 

The data presented here, while not definitive, permit some speculation on the mechanism of this apparent recovery of the irradiated PTFE. First note that unirradiated PTFE decomposes thermally at about 500 C by unzipping to release the monomer (16). Although the 31/69 ratio and the relative mass 81 intensity observed (Figure 6(a)) were much lower than that from the monomer (Table I), the gas produced here was almost certainly a mixture of saturated and unsaturated fragments. It seems likely that the unsaturated gas was a result of decomposition of the irradiated PTFE. Since unirradiated PTFE is thermally stable well above 300 C, it was presumably the damaged network that was susceptible to degradation in the 200 C to 300 C range observed here. 

A number of small unsaturated molecules or functional groups have been found to add across the M-M triple bond in Cp2Mo2(C0) to give adducts Cp2Mo2(C0) (un), where un = acetylenes ( [), allenes (42), cyanamides (43) and thioketones (44). In all of these adducts, the unsaturated fragments act as 4-electron donors to the dimetal center, thus reducing the M-M bond order from three to one. 

The condensation reactions of carbonyl metallates with neutral species, which are either coordinatively unsaturated or which will readily generate coordinatively unsaturated fragments, have also yielded a variety of mixed-metal clusters. The square-based pyramidal dianion [Fe5C(CO)i4] , for example, has been shown to react with a number of such species to yield octahedral FcjMC cluster compounds (Scheme 9) 269, 381). In some cases. 

The retro-ene reaction cleaves an unsaturated compound into two unsaturated fragments. A common example of a retro-ene reaction in organic synthesis is the acid-catalyzed decarboxylation of a (B-ketoester. The ester is hydrolyzed to the (3-ketoacid by the aqueous acid, which rapidly loses carbon dioxide to form enol. The loss of CO2 drives the reaction to the right-hand side. The enol rapidly tautomerizes to the methyl ketone (Scheme 8.17). 

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