Intermediate five coordinate carbonyls

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

Casey et al. have studied the decarbonylation reactions of [cis-(OC)4M(MeCO)(PhCO)], in which M is Mn or Re (16,17). These complexes lose a carbonyl ligand to form five-coordinate intermediates of the type [(OC)3M(MeCO)(PhCO)]. Reversible methyl migration proceeds much more rapidly than does phenyl migration. In the course of these studies, a phosphine substituted rhena-/3-diketonate complex, [fac-(OC)3(Et3P)Re(MeCO) (PhCO)], was prepared. 

Iodide and acetate salts increase the rate of reaction of Li [1] with CH3I at 25 °C in acetic acid. The effects of water, LiBF4, and other additives are also reported. Iodide salts also promote catalytic methanol carbonylation at low water concentrations. In the case of Lil promoter, lithium acetate is produced. The promotional effects of iodide and acetate on both the model and catalytic systems are rationalized in terms of iodide or acetate coordination to (1) to yield five-coordinate RhI anions as reactive intermediates for rate-determining reactions with CH3I.11... 

X-ray structure analyses of Rh(COCH3)(I)2(dppp) (14) and [Rh(I I)(I)(//-I)(dppp)]2 (15), where dppp l,3-bis(diphenylphosphino) propane, were reported. Unsaturated complex (14) possesses a distorted five-coordinate geometry that is intermediate between sbp and tbp structures.69 Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. By using labeled methyl formate (H13C02CH3) it was shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate.70... 

The same workers (46) report that HCl adds irreversibly to an Rh-olefin system when carbonylated alcoholic solutions of RhCls are refluxed with one mole of vp. The product is assigned a structure as in Fig. 35, the metal being in the - -3 oxidation state. The five-coordinate RhCl(CO)2 (vp) is held to be an intermediate in the formation of the Rh(III) system. 

Another example of a Bill compound is offered by the ligand VAPOL ( vaulted bisphenanthrol), which combines with AICI3 to form a three-coordinate complex, with no dative interactions. This compound will catalyze Diels-Alder reactions [64]. A five-coordinate compound, with two dative carbonyl bonds attached to the aluminum, has been postulated as the active intermediate in the catalytic reaction. 

The insertion of CO into metal-carbon a bonds has been reviewed.585-590 Carbonylation of alkyl platinum(II) complexes usually requires elevated temperatures, although at higher temperatures the reaction is reversible (equation 211).591 With PtMe2(dppe) insertion occurs into only one of the Pt—Me bonds. For complexes PtX(Ar)L2, carbonylation follows pseudo first-order kinetics. Rates are decreased by addition of L to a maximum value where the carbonylation rate is independent of L. The pathway involves formation of a five-coordinate intermediate PtX(Ar)(CO)L2, followed by dissociation to form PtX(Ar)(CO)L. The migratory step to yield PtX(COAr)L is unaffected by added L. This pathway is outlined in Scheme 6.502... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

Infrared spectroscopy and 13CO experiments show that in this reaction a five-coordinate nickel carbonyl complex with a v(CO) band at 2127 cm 1 forms as an intermediate. In this intermediate, the azide ligand, which had been inert in the [Ni(N3)(S3)] starting complex, evidently is labilized so much that it spontaneously dissociates N2 to form the NCO ligand of the final [Ni(NCO)-(83)] complex. Scheme 28 illustrates the suggested reaction mechanism. 

This rather surprising result is explained by recognizing that the 16-electron intermediate is five coordinate and could equilibrate the cis and trans carbonyls. A fluxional five-coordinate intermediate see Fluxional Molecule) would allow for enrichment in the axial position without dissociation of the axial CO. Thus, the stereochemistry of CO dissociation is consistent with expectations. 

An associative mechanism for carbonylation of iodobis(triphenylphosphine)aryl-platinum(II) complexes has been suggested. The carbon monoxide insertion reaction of PtI(Me)(CO)(PPh3), promoted by tertiary arsines or by SbPha, involves the intermediacy of the three-coordinate intermediate PtI(COMe)(PPh3). The nature of the intermediates in the carbonylation of trans-PtX(PhXPR3)2 was investigated and found to involve formation of a five-coordinate complex ". ... 

Studies of the base hydrolysis of cis-[Co(en)2(GlyOR)X] ions led to interesting developments. Base hydrolysis of halopentamine complexes of cobalt(III) occurs by an SnICB mechanism leading to a five-coordinate intermediate. In the N-bonded ester complexes of the [Co(en)2(GlyOR)Cl] type, the ester carbonyl group and solvent water can compete for the vacant site in the five-coordinate intermediate (Scheme 5) giving rise to the chelated glycine ester species and the hydroxypentamine respectively. 

Group 10 metal alkyls having d electrons may undergo the CO insertion from a four- or five-coordinate intermediate [8]. In a detailed kinetic study concerning the carbonylation, Garrou and Heck proposed two possible insertion pathways via five-coordinate adducts, [MR(CO)(X)L2] (M = Pt, Pd, Ni R... 

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