Transition metals carbonyl bridge

There are two principal modes of coordination of the carbon monoxide ligand in transition metal carbonyls terminal coordination, to a single metal atom, and bridging coordination, to two or more metal atoms. The Ols spectrum of Co4(CO)12, shown in Fig. 5, can be readily deconvoluted into two peaks corresponding to these two types of carbonyl groups109. This spectrum is useful for determining the relative chemical shifts for the two types because... 

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... 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

It is also relevant to record that several iron-carbonyl complexes with bridging, and in one case terminal, aryltellurol ligands have been prepared by reaction of Fe(CO)5, Fe(CO)12 or [ji-CpFe(CO)2]2 with diaryl ditellurides and which, together with complexes containing other transition metal carbonyls, e.g, ruthenium, osmium and manganese, provide a substantial number of interesting compounds.2... 

This method has been more widely used for the synthesis of bridged germylene complexes than for their silicon analogs. The reactions of transition-metal carbonyl monoanions with Me2GeCl2 give A-type bridged... 

The shielding tensors for O and " C in the CO molecule are anisotropic with zero asymmetry, reflecting the axial symmetry of the molecule. The isotropic shift of C with respect to TMS is 185 ppm, and the anisotropy in the solid is 406 ppm. Upon linear coordination in transition metal carbonyl molecules, these values typically change by tens of ppm 103). In the few bridge-coordinated compounds for which data are available the isotropic shifts are around 230 ppm and the anisotropies 175 ppm. (No data for solid-state platinum carbonyls have been reported, but there is no reason to believe that they are an exception.) In the CO molecule, the isotropic shift of O is 350 ppm with respect to H2O, and the anisotropy is 675 ppm. Again, these values change by a few tens of ppm in solid molecular metal carbonyls with linear coordination 104). 

The silylene migration may be involved in some stoichiometric reactions of hydrodisilanes with transition-metal complexes, forming disilanyl complexes through activation of Si-H bonds. Binuclear Ru and Os complexes 39 having terminal silyl groups and silylene bridges are isolated in the reactions of pentame-thyldisilane with Ru3(CO)12 and Os3(CO)12, respectively, albeit in low yields (Eq. 16) [33]. Similar silylene migration may be involved in the reactions of hydrodisilane with other transition-metal carbonyls such as Fe2(CO)9 and Co2(CO)8 [34]. 

Interest in the synthesis of transition metal carbonyl clusters incorporating interstitial main-group atoms is because they provide a conceptual bridge between or-ganometallic chemistry and the areas of inorganic solid-state and surface chemistry. In addition to serving as useful models for either solid-state binary alloys or for the chemisorption of heteroatoms on the step-site of a metal surface, these discrete molecular clusters are often markedly more stable, especially toward the temperatures and pressures required for catalytic reactions. 

Tin(II) bis(jS-ketonolates), Sn[0CRCHCR (=0)]2 (R, R = Me, CFj, Ph), are also excellent donor molecules to transition metal carbonyl residues, readily forming the type D structure chromium, molybdemun and tungsten complexes (8) under photolysis in tetrahydrofuran, as well as the manganese complexes (9). The cobalt complexes (10) and (11) prepared by the same method are somewhat different, and in these the stannylene functions as a bridging group. The two platinum(0) complexes (12) and (13) have been obtained from the reaction of tin(II) bis(pentane-2,4-dionate) and Pt(C2H4)(PPhj)2 under different conditions. ... 

A number of review articles have described the applications of multinuclear NMR methods for studying structures and internal rearrangements of metal clusters. The more specific cluster types covered were transition metal carbonyl clusters/ chiral clusters/ and mixed metal clusters containing carbyne or ketenylidene bridges. ... 

Bifurcation at the acceptor site is another structural feature which is common in certain transition metal carbonyl complexes. In this instance two C-H groups point towards a single oxygen atom of a carbonyl ligand. Bifurcation is not restricted to terminal CO ligands but has also been observed with bridging carbonyls (Figure 5.6). There are also examples of trifurcation. 

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