Back-bond state

There are two important effects of metal deposition on the electronic structure of the silicon surface. The first is the saturation of the dangling bonds and removal of the associated back bond states by a very small metal coverage so that the Fermi level is not pinned by dangling bond... 

Fig. 5.2-3ir Theoretical surface band structure of Si (100)2 X1, obtained with the asymmetric dimer model (see Sect. 5.2.2.3, Fig. 5.2-7). Ddown and Dup refer to the DB bands at the down and up atoms. The bands labeled Si - S5 and Bi -B5 are back-bond states (modified by the surface) [2.53]...

Th e ability to perform m oleciilar orbital (MO ) calculation s on m et-als is extremely useliil because molecular mechanics methods are gen erally unable to treat m etals. This is becau se m etals h ave a wide range of valences, oxidation states, spin multiplicities, and have 1111 usual bonding situations (e.g.. d%-p% back bonding). In addition. the 11 on direction al n at are o ( m etallic hon din g is less am en a-ble to a ball and spring interpretation. 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

TABLE 2.4. "Back of the Envelope" Estimation of the Energies of Valence-Bond States of the X- + CH3X- XCH3 +X- SN2 reaction ... 

Tertiary phosphines - and the analogous arsines - are able to stabilize transition metals in a variety of oxidation states and coordination geometries. Investigations of complexation with P-ligands were promoted by the high stabilization of metal by P ligands, which is mainly due to n-back bonding. 

All the inconsistencies can be traced back to the often incompatible requirements of electron-pair covalency as the manifestation of the ideal covalent bonding state on the one hand, and covalency as the opposite to ionicity on the other. On the basis of the ideas advocated in Section III. 1, it is comparatively easy to choose between the two alternatives and give the definition ... 

There is some uncertainty whether this complex should be described as [Vm(bipy- )3] or as [V°(bipy)3], In fact, given that 2,2 -bipyridine can act either as a cr-donor or a n-acceptor, the metal-ligand bond in these complexes is constituted by a cr-bond between the lone pair of electrons of the nitrogen atom and an unoccupied s-orbital of the metal. Such electron donation, increasing the electron density on the metal, can in turn favour a back-bonding from the d-orbitals of the metal and the unoccupied rt -orbitals of the aromatic pyridine ring. In short, if the metal ion is in a high oxidation state pyridine will act as a a donor, whereas if the metal is in a low oxidation state pyridine will act as a n acceptor. 

When a metal atom donates electron density to a bound ligand, usually by means of Ji-back bonding, electrophilic substitution reactions may be promoted. This is observed then usually with metals in low oxidation states and is therefore prevalent with organometallic complexes - and less with those of the Werner-type, where the metals are usually in higher oxidation states. Nevertheless there have been detailed studies of electrophilic substitution in metal complexes of P-diketones, 8-hydroxyquinolines and porphyrins. Usually the detailed course of the reaction is unaffected. It is often slower in the metal complexes than in the free ligand but more rapid than in the protonated form. 

---