Homoleptic carbonyls

Hydrogenation of C02 to formic acid could potentially proceed first by reduction to CO, followed by a reaction between CO and water to give formic acid, a reaction which is known (Eq. (3)). It is unlikely that this pathway to formic acid is common because very few homogeneous catalysts (primarily homoleptic carbonyl complexes) [71-73] have been reported for the hydrogenation of COz to CO, and because the few C02 hydrogenation catalysts that have deliberately been exposed to CO, in order to check whether this pathway is operating, have been poisoned as a result [18, 19, 31, 74]. 

Table 7.1 Calculated first BDEs De (Do) (kcal/mol) of neutral homoleptic carbonyl complexes. ...

In the mid-1960s, Dessy and coworkers [12, 13] provided an extensive survey of the anodic and cathodic reactions of transition metal organometallic species, including binary (homoleptic) carbonyls, and this provided a stimulus for many later detailed studies. Whereas the electrochemistry of heteroleptic transition metal carbonyls is covered elsewhere in this volume, that of the binary carbonyls, which is covered here, provides paradigms for the electrochemistry of their substituted counterparts. A key aspect is the generation of reactive 17-electron or 19-electron intermediates that can play key roles in the electrocatalytic processes and electron-transfer catalysis of CO substitution by other ligands. 

The first homoleptic carbonyl cations of Group 7 metals with the general formula [M (CO)6]+ (M = Mn, Tc, Re) were generated by Fischer and co-workers992 993 and Hieber and co-workers994 995 in the 1960s"6,997 applying halide abstraction by Lewis acids [Eq. (4.236)]. 

Figure 4.4. Metals with known homoleptic carbonyl cations (shading indicates structural characterization).

The homoleptic carbonyl salt [Hg(CO)2]+(Sb2Fn )2 reacts with traces of water in the HF—SbF5 superacid to yield the bis-aqua solvate [Hg2(OH2)2]2+(SbF6-)2-4 HF.1024 The cation is almost linear with nearly C2/, symmetry. The Hg—Hg and Hg—O bond distances are 2.4917 A and 2.148 A, respectively, and the O—Hg—Hg bond angle is 177.6°. 

Homoleptic carbonyl ligands, in palladium complexes, 8, 197 Homoleptic chromium alkyl compounds, preparation,... 

J.P. Kenny et al., Cobalt-cobalt multiple bonds in homoleptic carbonyls Co2(CO), (x 5-8) structures, energetics, and vibrational spectra. Inorg. Chem. 40, 900-911 (2001)... 

Photochemical reactions of the homoleptic carbonyls of manganese and rhenium with donor ligands have been thoroughly studied in recent years (2-4). The nature of the primary photoproducts of the complexes M2(CO)10 (M = Mn, Re) has been investigated by different techniques (151-165). The... 

In this work the ultrafast photophysical processes associated with photoinduced CO loss will be described with particular regard to the mononuclear homoleptic carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4. These complexes were the earliest metal carbonyls to be synthesized, indeed Fe(CO)5 and Ni(CO)4 have been known since the latter part of the nineteenth century. The carbonylation of nickel provided an important route to high purity nickel which was required by many important industrial processes. However, it is only within the last few decades that adequate models have been developed to explain the mechanisms of photoinduced CO loss from metal carbonyl complexes. 

The structures and properties of the binary (homoleptic) carbonyl complexes of the transition elements, including representative synthetic routes... 

Figure 3.4 Homoleptic carbonyl cations of the late transition metals...

The simplest homoleptic carbonyl species is Fe(CO)5. The 18 electron complex is easily made by direct combination of highly dispersed metal and CO at high tempera-... 

The majority of these contain CO ligands. The neutral homoleptic carbonyls have not been isolated, but anionic M(CO)6 as well as highly reduced M(CO) species are known. The original synthesis of the -1 species required elevated temperatures and high pressure but recently two simple, atmospheric pressure methods have been developed. They involve reduction of pentahalides in pyridine with Zn/Mg or in dimethoxyethane with sodium naphthalenide under an atmosphere of CO. These yellow salts contain discrete M(CO)6 anions. The facile syntheses of the octahedral hexacarbonyl anions allowed systematic exploration of the previously difficult to access area of low-valent complexes of Nb and Ta. Since M(CO)s are rather inert towards displacement of CO the substitution products of general formula M(CO)6- L have to be obtained by other routes, for example, by reduction of MX(CO)6- L compounds. The monosubstituted derivatives are conveniently prepared via the following method87 ... 

Some compounds and their reactions have been noted in Section 18-G-2. The homoleptic carbonyls are clusters such as M4(CO)i2 and M6(CO)i8. The hydride, HRh(CO)4, is very much less stable than HCo(CO)4 and has been made only under ca. 1400 atm pressure since it readily loses H2 to give clusters. Both Rh and Ir give anions [M(CO)4] and [M(CO)3]4 as R3NH+ salts as well as various cluster anions such as [Ir8(CO)22]2 and [Rh5(CO)i5] . Hydrido and other substituted polynuclear carbonyls are known. 

The majority of homoleptic carbonyls and many other derivatives can be formed with PF3 hgands instead of CO. In many instances, CO is exchanged by PF3 under pressure. In contrast to the elusive nature of (3), its PF3 analog HCo(PF3)4 is of very high thermal stability and does not decompose below 250 °C. Its acid base properties seem to resemble those of the carbonyl hydride, however. 

The homoleptic carbonyl (see Homoleptic Compound) Co3(CO)9 is not known, but a few substituted derivatives have appeared in the literature. The best characterized cluster seems to be Co3(CO)9H, which is found in 10% yield as a decarbonylation product of Co3(CO)ioH ([CosCJOH). The X-ray structure has disclosed a regular triangle with three bridging and six terminal CO groups. The hydrogen atom was not located in the diffraction study. 

The catalytic implications of the alternative coordination modes of nitric oxide in transition metal complexes were first noted by Collman (12). He argued that the linear bent transformation, concomitant with a change in the formal oxidation state of nitrogen from (III) to (I), results in the withdrawal of electron density from the metal center and facilitates the coordination of another ligand into a vacant site. Thus, the mixed carbonyl nitrosyl complex [Co(CO)3(NO)] undergoes thermal CO substitution by an associative mechanism, whereas the iso-electronic, homoleptic carbonyl [Ni(CO)4] reacts by a dissociative pathway (13). 

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