Silver Carbonyl Cations

In Table 5 are also shown the 0 K stepwise Ag+—CO bond dissociation enthalpies of Ag(CO)4+ from Armentrout and his coworkers . From their colhsion-induced dissociation experiments in a guided ion beam mass spectrometer, one finds a nonmonotonic change in Do[Ag(CO) CO] as a function of n. That the dissociation energy increases from n = 0 to n = 1 has been explained in terms of 4s-3da hybridization Z)o[Ag+—CO] is small because it is reduced by the energy needed to hybridize the metal ion, but Do[Ag(CO) —CO] assumes a more conventional value because the Ag(CO)+ fragment aheady has the metal in the required hybridization. In turn, Z)o[Ag(CO)+—CO] Do[Ag(CO)2 —CO] because the third carbonyl Mgand imphes loss of the 4s-3da hybridization. FinaUy, Do[Ag(CO)2+-CO] Do[Ag(CO)3+-CO] is explained by an increase of hgand repulsion. 

In Table 5 are also shown the 0 K stepwise Ag+—CO bond dissociation enthalpies of Ag(CO)4+ from Armentrout and his coworkers49. From their collision-induced dissociation experiments in a guided ion beam mass spectrometer, one finds a nonmonotonic change in ZJo[Ag(CO)n+—CO] as a function of n. That the dissociation energy increases from n = 0 to n = I has been explained in terms of 4s-3d r hybridization  

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Tetracarbonyl cations [Cr(CO)4(L2)]+, where L2 represents arylphosphines, alkyl, or aryl phosphites, or a bidentate ligand with phosphorus or arsenic donor atoms, have been produced by both chemical and electrochemical means. However, oifly one of these complexes, namely, tra i -[Cr(CO)4(PPh3)2]+, is stable enough to be isolated as the perchlorate salt. Exposure to light and moisture produces tra i -Cr(CO)4(PPh3)2 via disproportionation. Just as easily isolated, but somewhat less sensitive, are the [Cr(CO)3(PR3)3]+ cations. " Oxidation with a silver ion or NO+ converts the /uc-(R3 = Me2Ph, (OMe)3) and mer-(R3 = (OMe)3, (OMe)2Ph, (OPh)3) complexes into Cr products with mer structures. Light and heat promote the formation of reduced wer-Cr(CO)3 (PR3)3. These tetra(carbonyl) and tri(carbonyl) cations have been the subject of ESR spectroscopy and theoretical study. 

On the descriptive side, previously known binary carbonyl cations are usually of the [M(C0)6] type with M = Mn, Tc or Re (82), The oxidation state of the metal in these or other ternary cations is 0 or +1, and the ionic charge of the complex does not exceed +1. In addition, far more basic anions are used as counter ions. The effective atomic number rule, which plays an important role in judging stability, structure and reactivity of transition-metal carbonyls, is not valid for the noble-metal carbonyl compounds reported so far. The silver(I) and gold(I) carbonyl derivatives have 14, and the Pt(II) carbonyls have 16 electrons in the metal valence shell. 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

A mechanism involving the generation of a cationic alk(en)ylzirconocene (I X = R ) through chloride abstraction by silver(I) has been postulated (Scheme 8.22). This cationic intermediate is capable of activating the carbonyl group towards addition (II). Irrespective of whether an alk(en)yl group is added intra- or intermolecularly, a new cationic species is generated (either by R -transfer or Cl-abstraction from III) and the reaction thus proceeds in a zirconium-catalyzed manner [50]. 

The gold analog, [AujCOjj] is reported to be formed (with an [Sb2Fu] counterion) only under 100 bar CO [107]. The y(CO) bands of these species occur significantly above the stretching frequency of free CO (2143 cm ) for example the silver and gold tricarbonyl cations have IR absorptions at 2192 and 2212 cm respectively. This is taken to indicate that there is little or no metal —> CO Ti-backdonation in these species, which are termed non-classical metal carbonyls [109]. 

This reaction is widely utilised in organic synthesis, when carbonyl groups may be protected as the thioacetals or thioketals. Unlike acetals or ketals, the thio compounds do not undergo acid catalysed hydration, and may be used in acidic reaction conditions. The metal-directed hydrolysis is rationalised in terms of the soft-soft interaction of the sulfur with the metal cation, in contrast to the hard-soft interaction with a proton. Hydrolysis is readily achieved on treatment with aqueous mercury(n) or silver(i) salts. Once again, the... 

This transformation was further studied and the catalyst load could be decreased to 0.2 equiv.35 33 A mechanism was proposed through deuterium incorporation experiments, and the conclusion was that there was no 1,2 shift of the deuterium present in the starting material (A, Scheme 5.3) since exclusive formation of furans D deuterated on position 3 could be explained by the presence of an external source of deuterium (such as D20). Therefore, it is believed that after silver(I) coordination to the allenyl system (A, Scheme 5.3), the attack by the carbonyl oxygen may lead to an oxo cation intermediate B. Finally, proton lost would generate silver furan C that would lead to furan D after silver release (Scheme 5.3).39... 

The silver(l) carbene complex was used in the ROP of (L)-lactides [120]. Mechanistically, it is thought that the silver cation interacts with the carbonyl oxygen atom of the cyclic lactide. One of the carbene units then nucleophilicly attacks the carbonyl carbon atom resulting in C-O bond fissure and coordination of the oxygen atom on the silver cation. The role of the initial Ag-0=C interaction is to bring the lactide into position for nucleophilic attack by the carbene and to activate the carbonyl carbon atom for this attack (see Figure 4.40). 

The oligomerization and cooligomerization of conjugated dienes are representative reactions that proceed via transition-metal Jt-allyl intermediates. When (CsMesjRuCljt/ -butadiene) in dichloromethane was treated with an acetone solution of an equimolar amount of silver trifluoromethanesulfonate (AgOTf) in the presence of excess butadiene at ambient temperature, after which the mixture was allowed to react with carbon monoxide (1 atm), a cationic 1,5-cyclooctadiene carbonyl complex, [(C5Me5)Ru(CO)( -l,5-C8Hi2)]OTf, was isolated in 95% yield (Eq. 

An interesting but rather unusual reaction involves the direct carbonylation of carbocations to carboxylic acid derivatives. Carbenium ions can be generated from alkenes or alkanes in strong acidic media. Thus, tertiary carboxylic acids can be produced from C4 or higher alkenes Koch-Haaf reaction) [39] (e.g., eq. (8)). Interestingly, Koch carbonylations are known to be catalyzed by copper or silver cations [40]. 

The first step of the Provost reaction is the reaction of the alkene with iodine to form the cyclic iodonium ion. Next, the iodonium ion is stereospecifically opened by the silver carboxylate to form the corresponding frans-1,2-iodo carboxylate. The iodine is displaced intramolecularly by the carbonyl group of the carboxylate (anchimeric assistance) to form a cyclic cationic intermediate. In the absence of water, this cation is opened with the inversion of configuration by the second equivalent of silver carboxylate to afford the frans-1,2-dicarboxylate. However, in the presence of water Woodward-Brutcher modification) the common intermediate is converted to a c/s-orthocarboxylate which is hydrolyzed to the corresponding c/s-1,2-diol. 

Cationic bis(yhde) complexes [Ag(ylide)2]+ can be isolated only with aryl substituted ylide ligands of the formula Ph3P=CHR. They have been synthesized by the reaction of silver salts AgX (X = Cl, CIO4, N03) and free ylides in a 1 2 molar ratio. A large variety of different simple ylides (R = H, Me, CH(CH3)2)11,13 and carbonyl stabilized ylides [R = C(0)Me, C(0)Ph, C02Me, C02Ph]14 have been employed in this reaction. 

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