Resonance structures 1.3-diene complexes

In the crystal structures of the La3+ and Eu3+ complexes of Ln( 18-DTPA-dien) oligomerization was observed [20]. The coordination of the Ln3+ ion by each DTPA unit, however, was similar to that in the linear bisamides. Vapor pressure osmometry indicated that oligomerization in solution does not occur under the conditions applied (0.2-0.5 M). Because of the constraints imposed by linking the two amide functions in the macrocycle, the isomers 1,1 and 3,3 depicted in Fig. 1 are sterically very unfavorable. In line with this, resonances for only two pairs of enantiomers were observed in the 11 spectra of the Ln( 18-DTPA-dien) complexes. The dynamic behavior showed that one pair was rather static, whereas the other was rapidly interconverting. The AG for the interconversion (65 kj mol-1) was about the same as those observed for the SS AA isomerization in other DTPA-derivatives. In the Ln( 18-DTPA-dien) complex such an isomerization is only possible between mirror images 2 and 2. All other isomerizations require decoordination and, therefore, are much slower. 

C2—C3) and 141 pm (C3—C4). Just as for alkene coordination, we can draw two limiting resonance structures (24.25) for a buta-1,3-diene (or other 1,3-diene) complex. ... 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

The above type of bonding is assumed to occur in other metal-olefin and metal-acetylene complexes (172). Acetylenes have two mutually perpendicular sets of ir-orbitals and are therefore capable of being bonded to one or to two metal atoms both types of complexes are known. When the hydrocarbon is a nonconjugated polyolefin e.g., cyclo-octa-1,5-diene, each C C bond interacts independently with the metal atom. In complexes of conjugated polyolefins, e.g., cyclopentadiene, infrared and nuclear magnetic resonance studies (99) indicate that it is not yet possible to distinguish between structure (IV), in which each C C bond independently contributes two --electrons to the metal-olefin bonding, and structure (V), in which... 

Use. The oligomerization of olefins has generally been carried out with zero-valent transition metal complexes (mononuclear catalysts) and usually leads to an array of dienes see 1, 259). Schrauzer et at.1 of the Shell Development Co. reasoned that a dinuclear catalyst such as ZnfCo(CO).,J2 in which the two cobalt centers are connected close to each other will lead to new transition state formation from which different products can form. As a model, they examined the dimerization of nor-hornadiene and with the new catalyst obtained in almost quantitative yield a single dimer, m.p. 65-65.6°, shown unequivocably by elemental analysis (C14H16), infrared, nuclear magnetic resonance, and mass spectrometry to have the structure (2). 

For complexes of Tp and Tp some effort was expended in determining structural facets for both the diene and Tp ligand. It was thus concluded that the dienes adopt the anticipated rf-S-cis conformations, with an appreciable contribution from the metallacyclopent-3-ene resonance form, apparent from particularly low-frequency C-NMR chemical shifts for the terminal alkenic nuclei (ca. 3.9 ppm). However, since the /ch values were more typical of Ti-coordination, this representation was adopted. In respect of the Tp ligands, considerable fluxionality was observed, the rate of exchange being found to be heavily dependant upon the... 

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