Bridging carbonyl

The diarsine and arsine/phosphine analogues of dppm have been used to prepare bridged diplatinum(I) complexes, in both cases with terminal chloro ligands.116,117 Both complexes react with carbon monoxide to produce carbonyl-bridged species. The mixed thio/phosphine ligand Ph2PCH2SMe (PS) also forms a diplatinum(I) complex by conproportionation of its dichloroplat-inum(II) complex with [Pt(dba)2].118 This dimer reacts with carbon monoxide to produce an unsupported dimer [PtCl(PS)(CO)]2 with the thioether arm of the Ph2PCH2SMe unbound. 

Reaction of dipyrrinones such as 242 with A,A -carbonyldiimidazole (CDI) have been used to synthesize carbonyl-bridged dipyrrinones such as 243, which are highly fluorescent compounds (Equation 66) <2002JOC2713>. 

Figure 3. Possible isomers of [Rh5(CO)HI]2 that involve apical iodide substitution. Each equatorial rhodium has one terminal CO and each equatorial edge is carbonyl bridged all these COs have been omitted for clarity. Key O, CO , I a, observed and b, not observed.

However, these reactions jure associated with terminal rather than bridging carbonyl groups. In metal "ketonic compounds, the center for attack is normally the metal. This may be rationalized in terms of additional delocalization of the positive charge from the carbon to the metal center and implies a "carbyne contribution to the carbonyl bridge, of the form discussed above for the methylated products, again emphasizing the possibility that in systems of this type reaction may occur at either the metal or ligand center... 

A few clusters of interest containing germanium and transition metals have been reported.136-138 Dimethylgermane was found to replace only the bridging carbonyls between cobalt in a mixed germanium/cobalt/iron cluster complex (Equation (107)), and replacement of the carbonyl bridging the iron metal centers was not observed.137 A similar reaction leads to replacement of a bridging carbonyl in a mixed cobalt/silicon cluster (Equation (108)).136... 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

In 1964, the transient existence of 4 was proved unequivocally by Fieser and Haddadin. - The Diels-Alder adduct of 15 and tetracyclone (19 the stereochemistry of the carbonyl bridge is not known with certainty) when refluxed in diglyme in the presence of 15 delivered 4, which was trapped as Diels-Alder adducts 21 (mp 265°C) and 22 (mp 175-176 C) these compounds were isolated in a ratio of 1 1.4 in nearly quantitative yield. The... 

Nitration of DDT and its dehydrochlorination product 1,1 -dichloro-2,2-di-(4-chlorophenyl)-ethylene led to the formation of bis(3-nitro-4-chlorophenylene) compounds containing 1,1,1-trichloroethane and carbonyl bridging groups [19,20]. These compounds were converted to the corresponding bis(3-amino-4-chlorophenylenes) l,l-dichloro-bis-(3-amino-4-chlorophenyl)-ethylene and 3,3 -diamino-4,4 -dichlorobenzophenone in accordance with Scheme 2.9 [5, 22, 24]. 

Table I also shows that the mean lengths of the carbonyl-bridged Ni2-Ni3 bonds in the central Ni6(CO)3(/i2-CO)6 fragment of the dianions (2.43 A for each of the Compounds, 1,2, and 3) and the trianion (2.42 A) are virtually identical with those of the carbonyl-bridged Nii-Nii bonds. Each of these carbonyl-bridged Ni-Ni bonds is considered to correspond to a normal electron-pair bond.

The highly covalent nature of transition metal carbonyls and their derivatives leads to the 18-electron rule being closely followed. The mononuclear species Ni(CO)4, Fe(CO)5, Ru(CO)5, Os(CO)5, Cr(CO)6, Mo(CO)6 and W(CO)6 obey this well and, if the formalized rules of electron counting are applied, so do the metal—metal bonded and carbonyl bridged species. Such compounds are therefore coordinately saturated and the normal (but by no means unique) mode of substitution is dissociative (a 16-electron valence shell being less difficult to achieve than one with 20 electrons).94... 

Often the carbonyl bridges occur in pairs or complementary sets over two or more metal atoms. Examples include Fe3(CO)12, (ij5-C5H5)2Fe2(CO)4 (bridged isomers) and Rh4(CO)12. In the first two, the bridging carbonyls... 

Cotton has discussed this phenomenon extensively (37). For complementary sets of more than two carbonyl bridges, the equilibration is also probably concerted. All of the carbonyls in Rh4(CO)u appear magnetically equivalent at room temperature, with bridge terminal interconversion occurring via the formation of an unbridged species whose structure is analogous to Ir4(CO)12 (39). 

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