M-ring bonding

A similar phenomenon is observed on comparing a symmetric stretching vibrations of 7c-bonded metallocenes and ionic cyclopentadienides. The data in Table 4-13 shows that the M-ring bonding decreases in the order Fe > Mg> K. 

Cycloalkenes that have trans double bonds m rings smaller than 12 mem bers are less stable than their cis stereoisomers trans Cyclooctene can be isolated and stored at room temperature but trans cycloheptene is not stable above -30°C... 

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

There is also some contribution of structures in which two M—C bonds are broien, with formation of a ring double bond and an unshared electron pair on the metal atom, with no electron transfer. These structures increase the bond number of the carbon-carbon bonds and decrease that of the metal-carbon bonds. 

The increase of the exocyclic C—C bond stretching frequency from 1208 cm in toluene to 1264 cm in the benzyl radical and the simultaneous decrease of the C—C ring bond stretching frequencies (from 1494 and 1460cm to 1469 and 1446cm , respectively) result from electron density delocalization in the benzyl system. Furthermore, the force constant value for the C—C bond in the C6H5CH2 radical (5.5 X 10 N m ) is between the values for the ordinary C—C bond (4.5 x 10 N m ) and the double C=C bond (9.0 X 10 N m ) and is close to the corresponding force constant in the allyl radical (5.8 x 10 N m ). 

O-M-O bond angles required. One can relate bite sizes (distances between the donor atoms) of strain-free chelate rings of different sizes to the dimensions of the chair form of the cyclohexane ring (39), which has the minimum steric strain possible for a cycloalkane. The bite size of strain-free four- and six-membered chelate rings both correspond to... 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

It is inappropriate in this survey to attempt to summarise in a short space the results of all the above treatments, but virtually all the calculations indicate that in M(Cp)2 species the 7r(ej) metal-ligand interaction is dominant in the metal to ring bonding. However, debate has largely been concentrated on two points, in the first place the extent of the validity (or otherwise) of Koopmans theorem, and, further, the question of the correct energetic ordering (5 < o < n or o < 5 < n) of the mainly metal tf-type orbitals. 

Recently, a metallocene/MAO system has been used for the polymerization of non-conjugated dienes [204, 205]. The cyclopolymerization of 1,5-hexadiene has been catalyzed by Zieger-Natta catalyst systems, but with low activity and incomplete cyclization in the formation 5-membered rings [206]. The cyclopolymerization of 1,5-hexadiene in the presence of ZrMe2Cp2/MAO afforded a polymer (Mw = 2.7 x 107, Mw/Mn = 2.2) whose NMR indicated that almost complete cyclization had taken place. One of the olefin units of 1,5-hexadiene is initially inserted into an M-C bond and then cyclization proceeds by further... 

In turn, each M—X bond engages in 3c/4e cu bonds with the opposite L donor of the other r 3-Cp ring. Thus, the following resonance structures summarize the primary bonding pattern, which can be compactly described as two tu bonds ... 

Table 9. Valence m.o. electron configuration Ofg e"g e%g, enthalpy of sublimation, AHSU, and standard enthalpy of formation, (AHf, g), of metallocenes. Mean bond dissociation enthalpy, D (M-Cp) (Refs.68 69 ) and (Metal-cyclopentadienyl ring) bond length, r(M-Cp). (Ref.72 ).

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