Metal electron density

Donation of metal electron density into the molecular orbital of CO. An analogous process may be drawn in the yz plane. 

This model was first used102 to investigate the dependence of the potential of zero charge on the metal. Only the bulk-metal electron density changes from metal to metal. For a metal electrode connected to a standard electrode to form an electrochemical cell, the potential of zero charge is given by Eq. (2) or, for a metal M,... 

It is interesting to note that the decrease in metal electron density that accompanies the change from five- to six-coordinate geometry does not have a detectable effect on the metal-carbene carbon bond length in these complexes. The metal-carbyne carbon bond in several osmium carbyne... 

The initial site of protonation in these reactions has not been unambiguously determined. Alkyl ligand formation by protonation at the metal followed by a rapid 1,2-proton shift to the alkylidene ligand is equally as plausible as direct protonation at Ctt. As the metal electron density... 

The osmium-carbyne carbon bond lengths for the three complexes do not differ significantly, and reference to Table IV indicates that these distances are distinctly shorter than the characterized metal-carbon double bonds of osmium carbene and carbonyl complexes. In both osmium alkylidene and carbyne complexes, then, the metal-carbon multiple bond lengths are largely insensitive to changes in the metal electron density (cf. Section IV,B). 

The similarity between the bonding models for transition metal carbene and carbyne complexes was noted in Section II. That the reactivity of the metal-carbon double and triple bonds in isoelectronic carbene and carbyne complexes should be comparable, then, is not surprising. In this section, the familiar relationship between metal-carbon bond reactivity and metal electron density is examined for Ru and Os carbyne complexes. 

Figure 5.32 Comparison of metal hydrides with complex hydrides [50]. Metal hydrides consist of an almost unchanged metal lattice with the interstitial sites filled with hydrogen. The additional electrons from hydrogen are added at the Fermi level of the metallic electron density of states and an additional hydrogen-induced band several eV below the Fermi level accommodates...

Figure 2.8. Electron density contours for atomic chemisorption on jellium with electron density that corresponds to A1 metal. Upper row Contours of constant electron density in the plane normal to the surface. Center row Difference in charge density between isolated adatom and metal surface, full line gain and dashed line loss of charge density. Bottom row Bare metal electron density profile. Reproduced from [30].

Lewis acids, such as BF3 and AlMe3, can also bind to metals directly (equation 29)53 although they often attack ligands such as CO and N2 (equation 30.)54 Attack at the metal raises v(CO) of attached carbonyls by reducing the available metal electron density, but attack at CO or N2 lowers v(CO) or v(N2) because the lone pair donated to the Lewis add has C—O and N—N bonding character. 

The key points from these experiments are that the more easily replaceable monophosphine ligands are required for the reduction of N2, which is favored by the presence of oxo-anions. Thus, as the reaction proceeds and electron density passes from metal to N2, the 7r-acceptor phosphines are replaced successively by 7r-donor oxo species. This change in ligand encourages further release of metal electron density onto the bound, partially reduced N2, which results in its protonation. This resulting effective increase in the oxidation state of the metal then causes further substitution of the softer phosphines by the harder oxo-anions. These mutually enhancing effects result ultimately in complete loss of all phosphine ligands and the production of NH3. 

As Beck and Nitzmann(74) observed, the specific intensities of the species V(CO)g , Cr(CO)g, and Mn(CO)6, which are, respectively, 947, 610 and 252 X 10 liter mole" cm as measured in tetrahydrofuran, show a rather striking linear dependence upon the formal oxidation state of the control metal atom. The same trend also occurs for the species W(CO)6 and Re(CO)6" , and it is taken to indicate the availability of the d metal electron density. In fact, there is now a considerable body of data available to support the general conclusion that the intensities of the carbonyl stretching vibrations of an isoelectronic and isostructural series of derivatives increase steadily from cations through neutral molecules to anions. For examples, compare the following pairs of species Ni(CO)4, Co(CO)4- (Table I) M(CO)5X, M (CO)sX- (Table II) w-M(CO)4X2, cw-M (CO)4X2 (Table III) and less rigorously M2(CO),o, HM2 (CO),o (Table VII). Beck and Nitzmann also concluded that the effect on intensities on going from a less polar to a more polar solvent increased from cation... 

Figure 2 Electron-density contours for chemisorption. Upper row contours of constant electron density in (any) plane normal to the metal surface containing the ad-atom nucleus (indicated by -f). The metal is to the left of the solid vertical line. Center row deformation charge density. The polarization of the core region, shown for Li, has been deleted for Si and Cl because of its complexity. Bottom row The bare-metal electron-density profile, shown to establish the distance scale. (From Ref. 38.)...

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