CO loss

Tungsten alkoxycarbene complexes underwent similar anti-syn rearrangements but were much less prone to undergo CO loss [5-10]. 

The sonolysis of Mn2(CO)10 makes for an interesting comparison (186), since either metal-metal (as in photolysis) (187) or metal-carbon (as in moderate temperature thermolysis) (188) bond breakage could occur. Ligand substitution will occur from either route producing the axially di-substituted Mn2(CO)8L2. Using benzyl chloride as a trap for the possible intermediacy of Mn(CO)5, the sonochemical substitution of Mn2(CO)10 has been shown to follow the thermal, rather than the photochemical, pathway of dissociative CO loss. 

Less work has been done on the time-resolved IR spectroscopy of Cr(CO)6 in the gas phase (69,91,92). Broadly, the results were similar to those with Fe(CO)5. Ultraviolet photolysis (249 nm) provides evidence for several CO loss products (92) tentatively assigned as Cr(CO)5 and Cr(CO)4. 

Church and co-workers (77) have obtained time-resolved IR spectra of both Mn(CO)5 and Mn2(CO)9 by flash photolysis of Mn2(CO)I0 in solution. The spectra (Fig. 11) were in close agreement with the spectra of matrix isolated Mn(CO)5 (22) and Mn2(CO)9 (5,106). There was a bridging vc 0 band for Mn2(CO)9 showing that it has a CO-bridged structure in solution as well as in the matrix. Structural information of this type could not have been obtained from uv-vis spectroscopy. Similarly, the IR spectra indicated that Mn(CO)5 had the same C4v structure in solution (77) as in the matrix (22). In solution (77), the yield of Mn2(CO)9 was approximately equal to that of Mn(CO)5. Bearing in mind that one molecule of Mn2(CO),0 produces two molecules of Mn(CO)5 [Eq. (14)], CO loss from Mn2(CO)10 [Eq. (15)], must be the major process at these photolysis wavelengths (37,77). 

C5Me5) was stable up to room temperature (108,109). A surprising feature of the matrix photochemistry (6,7) was that only the trans isomer of [CpFe(CO)2]2 was photolyzed, and it may be that this was the consequence of concerted CO loss from the trans isomer and isomerization to form the triply bridged product (6). 

In the absence of 13C hyperfine interaction measurements, it is not clear to what extent dissociative electron capture with CO loss occurs for such molecules. It is entirely possible, for example, that electron capture by M(CO)sI results in loss of the axial CO ligand, yielding the radical M(C0)4I2-. If so, it is noteworthy that CO is lost in preference to I-. 

Generally, the rheology of polymer melts depends strongly on the temperature at which the measurement is carried out. It is well known that for thermorheological simplicity, isotherms of storage modulus (G (co)), loss modulus (G"(complex viscosity (r (co)) can be superimposed by horizontal shifts along the frequency axis ... 

Figure 3.41 Integrated peak intensities for (a) CO gain (b) CO loss features and (c) total CO intensity compared to

The difference between these two sets of gas phase experiments is very interesting and a full explanation is awaited. Meanwhile it is worth commenting that matrix isolation work has already provided information about these lower carbonyl fragments. Prolonged photolysis of M(C0)6 (M Cr, Mo, W) produces sequential CO loss to give M(C0)5, M(C0)4, M(C0)3 and M(C0)2 ( U > with the... 

Organometallic Intermediates by CO Loss. There are many studies where the primary photoproduct generated by CO loss has been characterized in a matrix. One example will illustrate. 

By contrast, when (rr-C5H5)Fe(CO)2(COR) is photolysed (6 ), even in CO matrices, only CO loss is observed with (ri -C5H5)Fe-(CO) (COR) being formed en route to (ri5-C5H5)Fe(CO)2R ... 

As already noted, the carbonylation of bis(pentamethyl-cyclopentadienyl) actinide hydrocarbyls is irreversible in the cases studied thus far. Thus, thermolysis does not result in CO loss, but rather in interesting chemical reactions. Thermolysis of 1 ( 5) in toluene solution results in hydrogen atom migration to yield an enolate (eq.(5)). NMR studies establish that eq.(5) is essentially quantitative, and that the stereochemical course... 

Barrau and coworkers have synthesized a series of iron and ruthenium complexes by irradiation of Me2HGe(CH)KGeMe2H and Me2HGe(CH)K SiMe2H (n = 1, 2) in the presence of Fe(CO)5 and Ru3(CO)i293. In each case irradiation causes CO loss, with the formation of the M(CO)4 species (reaction 43). When n = 2 the products are photostable with n = 1 (65) a mixture of products (66-69) are obtained due to secondary photolysis (reaction 44). The mechanism, outlined in Scheme 23, is presented to explain these observations. 

Dotz reaction is proposed. According to our calculations the addition of the alkyne molecule to the carbene complex takes place before CO loss in the initial steps of the reaction. Further, our study shows that a novel proposal involving a chromahexatriene intermediate entails lower energy barriers and more stable intermediates than the previous reaction mechanisms postulated by Dotz and Casey. The novel findings query revision of the classically assumed paths and put forward that additional experimental and theoretical studies are necessary to definitely unravel the reaction mechanism of this intringuing reaction. 

This section describes the main results obtained in our studies of the Dotz reaction mechanism [26-29, 39]. The section is divided as follows First, the results for the initial part of the reaction (9—>13) are presented. The central discussion will be whether the alkyne binds the carbene complex after or before CO loss. Then, the results for routes A, B and C (Figure 3) are discussed. In particular, we will examine the suitability of the novel route C involving a chromahexatriene intermediate. 

Besides having different accurate mass, carbenium ions and acyiium ions fortunately exhibit different dissociation pathways allowing their differentiation from the appearance of a mass spectrum. Acyiium ions undergo CO loss (-28 u), thereby forming carbenium ions, but they do not eliminate hydrogen molecules ... 

Fig. 6.33. El mass spectrum of phenol showing an intensive peak due to CO loss at m/z 66. Spectrum used by permission of NIST. NIST 2002.

Note The above mechanism of CO loss from phenols is perfectly analogous to HCN loss from aniline and other aminoarenes (Chap. 6.14.2). 

Quinones [124] and aromatic ketones such as flavones, [116] fluorenone, anthra-quinone, and similar compounds [121] dissociate by competing and consecutive losses of CO and C2H2. Multiple CO losses may also occur subsequent to the RDA reaction of flavones. [116,125] As these molecules all have large 71-electron... 

CO Loss from Transition Metal Carbonyl Complexes... 

Whatever the mechanism of CO loss might be, in none of the above cases CO loss proceeds by a simple bond cleavage. Instead, multistep rearrangements are necessary to cut the CO moiety out of the precursor ion. 

Note The detection of CO loss from molecular or fragment ions usually indicates the presence of carbonyl groups. However, it is less indicative of molecular structure than the highly specific reactions discussed before, because a multitude of rearrangement processes can be effective. These might even lead to CO loss in cases where no carbonyl group exists, e.g., from phenols. 

The mass spectrum of aniline has been known since the early days of mass spectrometry. [122] Initially, the observed [M-27] ion has been interpreted in terms of HCN loss (Fig. 6.56a). The mechanism for loss of the elements of [H, N, C] from aminoarenes is perfectly analogous to CO loss from phenols (Chap. 6.9.1). [231] More recently, it could be demonstrated that loss of hydrogen isocyanide, HNC, occurs rather than losing the more stable neutral species HCN, a behavior typical of ionized pyridine. [222]... 

Example The FD spectrum of a ruthenium-carbonyl-porphyrin complex shows an isotopic pattern very close to the theoretical distribution (Chap. 3.2.8). The loss of the carbonyl ligand chiefly results from thermal decomposition. A spectmm accumulated close to BAT (scans 19-25, EHC = 25-30 mA) is nearly free from CO loss while a spectrum accumulated of scans 30-36 (35 0 mA)... 

Fig. 8.19. Thermal CO loss during FD measurement of a mthenium-carbonyl-porphyrin complex. Adapted from Ref. [101] by permission. M Publications, 1997.

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