7r-bonded ethylene

The double bond in ethylene contains one a bond and one 7r bond. The a bond forms from the end-on overlap of two hybrid orbitals, and the 7i bond forms from the side-by-side overlap of two atomic p orbitals. Figure 10-21 shows the complete orbital picture of the bonding in ethylene. Ethylene is the simplest of a class of molecules, the alkenes, all of which contain CDC double bonds. The alkenes are the subject of our Box on page 404. 

In the initiation reaction, one n eiectron of the ethyiene moiecuie pairs with the singie eiectron on the oxygen atom of the free radicai. Together, these two eiectrons create a new C—O a bond (shown highlighted in purple). The second electron from the ethylene 7r bond remains as an unpaired eiectron on the outermost carbon atom of the product. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The band at 1600 cm-1 due to a double-bond stretch shows that chemisorbed ethylene is olefinic C—H stretching bands above 3000 cm-1 support this view. Interaction of an olefin with a surface with appreciable heat suggests 7r-bonding is involved. Powell and Sheppard (4-1) have noted that the spectrum of olefins in 7r-bonded transition metal complexes appears to involve fundamentals similar to those of the free olefin. Two striking differences occur. First, infrared forbidden bands for the free olefin become allowed for the lower symmetry complex second, the fundamentals of ethylene corresponding to v and v% shift much more than the other fundamentals. In Table III we compare the fundamentals observed for liquid ethylene (42) and a 7r-complex (43) to those observed for chemisorbed ethylene. Two points are clear from Table III. First, bands forbidden in the IR for gaseous ethylene are observed for chemisorbed ethyl-... 

FIGURE 16.10 The structure and bonding in the anion of Zeise s salt. A cr-bond results from the overlap of a dsp2 hybrid orbital on the metal and the 7T orbital on ethylene. Back donation from a d orbital on the metal to the -k orbital on ethylene gives some 7r bonding (shown in (c)). 

FIGURE 21.8 The 7r bonding that occurs between ethylene and Pt in Zeise s salt. 

FlO. 2. The ethylene-platinum bonding (a), the common representation emphasizing w electron donation (6), the e-bonding molecular orbital (c), the 7r-bonding molecular orbital. 

Restricted Rotation. A little reflection will reveal that maximum overlap in a 7r bond is produced when the p orbitals are parallel to each other, and any rotation of these orbitals about the bond direction would decrease overlap. Since the directions of the p orbitals are fixed at right angles with respect to the cr bond planes, the most stable configuration of ethylene with maximum tt overlap is achieved when the cr bond planes coincide in other words, all the atoms in the molecule should be in one plane. This is indeed true in ethylene, and experiments indicate that rotation of the two CH2 halves about the C=C bond results in an increase in potential energy. There is restricted rotation in ethylene. Rotation about pure cr bonds is not restricted in this way. 

Most of the ground states of complexes seem to have structure XXIV, but XXV reasonably could provide a mechanism for rotation about the metal-olefin bond axis with a low energy barrier. Cramer (II) found that 7r-cyclopentadienylbis(ethylene)rhodium(I), XXVI, gave two broad signals (r = 7.23, 8.88 ppm) for the ethylene protons at —25° and that... 

The radius of charge circulation in the twisted molecule is not specified but should be of the order of the C—C bond length. In fact, using r = 117 pm, the calculated energy of 270 kJmol-1 agrees with the accepted 7r-bond strength in ethylene. 

M—has also been reported for olefins and acetylenes ir-bonded to rhodium and to platinum (6, 21, 46, 87). In the case of rhodium, iy(i°3Rh—is between 10 and 16 Hz for a 7r-bonded olefin (see Table XXVII), while for the cr-bonded carbon in [(C5H5)Rh(ff-C3Hs)-(w-CsHb)], 7( ° Rh—is 26 Hz. It was suggested the bonding of the olefin results from a 60% contribution from a dsp -vnet X orbital and sp -carbon orbital 21). For the olefins and acetylenes w-bonded to platinum 7( Pt—is between 18 and 195 Hz (see Table XXIX) compared to the range of 360 to 1000 Hz reported for carbon cr-bonded to platinum. It was found that 7( Pt— C) is less for a 7r-bonded acetylene than for a rr-bonded ethylene. This was considered as evidence for the Chatt-Dewar-Duncanson molecular orbital model 39, 63) of TT-bonding (XIV), rather than the formally equivalent valence-bond treatment, (XV) and (XVI) 46). However, no allowance appears to have been made for the effect on the hybridization at the carbon of the pseudo-... 

One of these bonds is ir-bond. It is formed by electron transfer from ligand to metal. Depending on their nature, such ligands can contribute a variable number of 7r-electrons two (ethylene), three (the 7r-allyl group), four (cyclobutadiene), five (the cyclopentadienyl group), six (benzene), etc. (see Figure 1). It means that 7r-bond exist between... 

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