Transition electron back donation

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

One more band was found in TS-la sample at 2126 cm below that of free CO in the gas phase. Frequency lowering is characteristic of d-electron back donation to the antibonding orbitals of CO, that is why this band can be considered as an evidence for the reduced transition metal sites on the surface. 

Our own quantum mechanical calculations point to a transition state for oxygen transfer to the olefin being partly stabilized by electron back-donation from an oxygen lone pair fo a n orbital on the C=C double bond. Of the two oxygen lone pairs, the one in the TiOO plane is already involved in O to Ti a donation and is therefore less available for C-O bond formation than the one perpendicular to the TiOO plane (see Fig. 13.5). 

Dialkyl H-phosphonates exhibit a number of complexation modes with transition metals depending on their type. In complexes of dialkyl H-phosphonates with some early transition metals (Zr, H) in their higher oxidation states, the phosphonate ligands act as lone-pair a-donors and are coordinated to the metal center through the phosphoryl oxygen atom. In contrast to this, the late transition metals (Ni, Pd, Pt, Co, Ir, Rh, and Ru) exhibit a well-expressed preference to the more soft phosphorus donor of the phosphite tautomeric form. The late transition metal complexes of dialkyl H-phosphonates are additionally stabilized by the electron back donation from the electron-rich metal center to the empty and 3a orbitals at the phosphorus donor. 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

Metal hydrides containing transition metal (TM)-hydrogen complexes, with the transition metal in a formally low oxidation state, are of fundamental interest for clarifying how an electron-rich metal atom can be stabilized without access to the conventional mechanism for relieving the electron density by back-donation to suitable ligand orbitals. By reacting electropositive alkali or alkaline earth metals ( -elements) with group 7, 8, 9, and 10 transition metals in... 

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

If back donation occurs to a ligand, the flow of electron density from the metal leaves less electron density to be donated in the opposite direction. It seems that this should have little effect on the donation of a pair of electrons on the ligand in the trans position to form a a bond. Accordingly, the major factor appears to be the stabilization of a five-bonded (trigonal bipyramid) transition state as a result of 7r bond formation. Ligands that readily form 1r bonds include some of those that generate the largest trans effect. 

Generally, the metals that form stable carbonyl complexes are those in the first transition series from V to Ni, in the second row from Mo to Rh, and in the third row from W to Ir. There are several reasons for these being the metals most often found in carbonyl complexes. First, these metals have one or more d orbitals that are not completely filled so they can accept electron pairs from a electron donors. Second, the d orbitals contain some electrons that can be involved in back donation to the k orbitals... 

Metal Preferences. LVC s are formed mainly by transition metals to the right in the periodic table (especially elements in Group 8). This is in part due to the availability of d electrons that can be used in back-donation to the 7r-accepting ligands. Moreover, the formation of LVC s is not particularly "row-sensitive" by which I mean that the first-transition-series metals, Fe, Co and Ni, tend to form most of the same cluster compounds as their congeners, Ru, Rh, Pd and Os, Ir, Pt. 

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

At the same time the third-row-element based molecule is un-satured and expected to display H -acid character. Thus the most propitious conditions for bonding to a transition metal should be those under which the metal can both accept and back-donate electron density to the P4 ligand. 

What is the effect of d orbitals on the binding energy It is known that transition metals bind hydrogen in two different ways (1) as an intact molecule (39) or (2) as the insertion product (40). The common explanation is that the type of binding depends upon the nature of M, i.e., its ability to back-donate electrons from filled... 

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