Carbonyl dissociation

Carbonyl dissociation will diminish in the presence of external CO. Carbon monoxide will also tend to trap out the unsaturated low-cluster fragments and therefore prevent cluster formation. The formation of Os5, Os8, Os7, and Os8 clusters from Os3(CO)12 is [CO]-dependent, the relative yields of the lower or higher clusters depending on the CO pressure. 

Using statistical-dynamical methods and transition state theory, Zhang and co workers demonstrated that excited carbonyls dissociate promptly to prodnce OH radicals (11%) or isomerize to form dioxirane (32%) or are collisionally stabilized (57%) . 

In view of the structural similarity between the Os(CO)4 units in Os(CO)4H2 and Os3(CO)i2 noted above, it is interesting to compare the Os(CO)4H2 thermolysis rate, for which we said carbonyl dissociation is rate determining, with the known rate of dissociative carbonyl exchange for Os3(CO)i2 (20). The exchange rate per Os(CO)4 unit extrapolated to 125.8°C is 25 X 10-5 sec-1 the rate of Os(CO)4H2 thermolysis at that temperature is 6 X 10-5 sec-1. The similarity of these numbers is final evidence that carbonyl dissociation from Os(CO)4H2 does occur in the rate-determining step. A vacant coordination site apparently is needed before the actual dinuclear elimination step can occur. 

Combination of both of the above elements in a single molecule such as Os(CO)4(H)CH3 gives rise to facile dinuclear elimination. The dihydride is capable of dinuclear elimination but must rely on the comparatively hign-energy process of carbonyl dissociation to provide the necessary vacant coordination site. The dimethyl compound has the necessary vacant site easily available but no hydride to interact with it. The hydridomethyl compound has both elements and is uniquely unstable. 

The oxygen end of the carbonyl ligand is a hard, albeit weak, base (48) and is thus compatible with binding H+. The major problem with this type of carbonyl activation is that because the M—CO bonding has been weakened via a reduced synergic interaction, carbonyl dissociation may become competitive. 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

The derivatives of chromium hexacarbonyl, Cr(CO)e (L)j , make up the single largest class of organochromium compounds, and a substantial number of these are Cr compounds as well. The most common synthetic route involves direct reaction of Cr(CO)6 and x L, but displacement of a weakly bound hgand from a carbonyl derivative is also frequently employed. Carbonyl dissociation is usually promoted with heating or UV irradiation. 

Chapter 13 gave a brief introduction to carbonyl dissociation reactions, in which CO may be lost thermally or photochemically. Such a reaction may result in rearrangement of the remaining molecule or replacement of CO by another ligand ... 

Ethyl rhenlumpentacarbonyl has been reacted with various metal hydrides in acetonitrile 158). The observed products were heterobimetallic compounds, although a solvated rheniumtetracarbonyl acyl complex was detected [Eq. (47)]. If the metal hydride is in excess, the rate-determining step is formation of the propionyl complex. The reaction was subsequently found to be first order in both the propionyl complex and the metal hydride. The second-order rate constants were measured and were found to be the reverse of the order of the acidities of the transition metal hydrides, which implies that the hydrides react as nucleophiles with the propionyl complex. In a separate experiment, [Re(COEt)(CO)j] was found to react with [Re(H)(CO)j] only after carbonyl dissociation, implying that the metal and not the acyl carbonyl is the site of nucleophilic attack by transition metal hydrides on acyl complexes. 

The photochemistry of M(CO>5X, X = halide, alkyl, hydride, etc., is dominated by carbonyl dissociation (see 13.2.4.1). In the absence of potential ligands, halogen-bridged dimers are formed ... 

Whereas most trinuclear carbonyls have photodeclusterification as their dominant reaction mode, carbonyl dissociation occurs in OsjfCOljj, e.g., stepwise replacement of up to three carbonyl ligands is possible ... 

Table 2 First metal-carbonyl dissociation energy (kJ/Mol) in a number of metal carbonyls8...

The intermolecular Pauson-Khand annulation using dicobalt hexacarbonyl-alkyne complexes met only with limited success, possibly because of the higher energy of the cobalt-carbonyl bond. In contrast, chromium carbonyls dissociate more easily, and the reactions of metallocarbenes of chromium, e.g., the Dotz annulation of alkynes, are improved by sonication. 

The mechanism of isomerization of yac-[Mn(X)(CO)3 P(OMe)2Ph a] to the mer-trans-form is most probably SnI, with phosphite rather than carbonyl dissociation as the rate-determining step. These conclusions are based on the observed first-order kinetics, the positive entropies of activation (Table 4), and the observation that the... 

Group VIII. Iron. Products of the reaction of tricarbonyl-(l,2,3,6-A -cycIo-octadiene)-iron (8) with phosphines depend on the nucleophilicity of the base. In the case of PPha and P(OPh)3, (9) is formed by carbonyl dissociation followed by addition of the base. This leads to the familiar rate law... 

Table 5 Calculated and Experimental First Carbonyl Dissociation Energies (kcal/mol) of M(CO)f, (M = Cr, Mo, W)... 

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