The 7r orbitals of

Formation of a <7-bond by donation from the 7r-orbital of ethene into a vacant metal dsp2 hybrid orbital... 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

IR data [131] shows a trend to increasing ifC—O) as the substituents on the phosphine became more electron withdrawing (Table 2.11) so that as the cr-donor power of the phosphine decreases and the 7r-acceptor power increases, the electron density at Ir decreases and electrons are removed from the 7r -orbital of CO [132]. 

The observed decrease of the CO-stretching frequency as the basicity of trans ligand L increases can be explained by enhanced backbonding through either increasing inductive a-donation of L (Case D, Fig. 1) or reduced competition of the 7r-acceptor orbitals in the pyridines with the 7r -orbitals of the CO ligand about the metal dff-electrons (Case E, Fig. 1). 

At least in bromoform, not the slightest trace of a dicarbonyl formed according to Eq. (2), M = Fe, could be observed. Obviously, the d -electron density at the Fe11 ion is not sufficient to feed the 7r -orbitals of two axial carbon monoxide molecules with the necessary electron density for bonding. In other words, the first jr-acceptor molecule labilizes a second one tram to itself according to Case E (Fig. 1) which is the inserve of Case C (trans-jr-donor labilization, Sect. 5.2). 

The HOMO of thiocarbonyl compounds corresponds to the n-electrons of sulfur the 7r-orbitals of the C=S bond usually... 

Following the proposals of Rooney et al. [85—87], it has generally been assumed that, as with monoolefins, the adsorbed state of an alkyne active in hydrogenation is a 7r-complex formed by the interaction of the 7r-orbitals of the acetylenic bond with two metal atoms. The 7r-complexed alkyne may be represented as structure L. 

Figure 10.20 Construction of the 7r orbitals of butadiene by end-to-end interaction of two ethylenes.

Let us suppose, for example, that we wish to know about the interaction of the 7r orbitals of butadiene with orbitals of some other molecule. If the butadiene is unsubstituted (Figure 11.2), the molecular symmetry and the symmetry needed for the orbital model are the same. The symmetry elements of the mole-... 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

There is only little information about ORD or CD of simple enamines. The ORD curves of (3V-pyrrolidyl)-5a-androst-2-en-17/ -ol acetate (109) and of 4-methyl-4-azaan-drost-5-en-17/ -ol acetate (110) were also investigated195. The enamine 109 shows a high absorption band in the UV spectrum at Amax 228 nm, with e 8000 (cyclohexane). The cyclic enamine 110 possesses a similar UV spectrum (Amax 221 nm, e 8500 in cyclohexane Amax 219 nm, e 8700 in ethanol). On the other hand, only the cyclic enamine 110 shows a high Cotton effect (Figure 7). The shape of this ORD curve indicates that the first Cotton effect is negative, and is then followed by a second positive effect. The remarkable difference in the ORD curves of two enamines 109 and 110, despite their similar UV spectra, may be traced to the differences in steric repulsion of the px orbital of the nitrogen atom, and the 7r-orbital of the double bond. Since free rotation along the C—N bonds in the enamine 109 is possible, the N—C=C— system becomes planar and the... 

The last example quoted deals with dehydrogenation, this time of larger molecules, studied by UPS for cyclohexane over Pd(lll) and Pd films. Rubloff et did not study catalysis but chemisorption. At r< 120K adsorption was without decomposition, but at 300 K dehydrogenation gave benzene as the surface species. This was suggested from comparative experiments in which benzene itself was adsorbed. It appears that the 7r-orbitals of benzene are involved in bonding. They found no evidence for the formation of cyclohexene or cyclohexadiene. Their work should be compared with that of Tetenyi et al., who identified such intermediates by radiochemical methods. 

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