Cycloheptatrienyl complexes bonding

Three zirconium/cycloheptadienyne complexes (231a-c) have been prepared by /3-hydrogen elimination from a mixture of cycloheptatrienyl complexes 269-271 (Scheme 33) and have been used as intermediates for the preparation of a zirconaazulene.87 The alkyne complexes are formed to the exclusion of the allene isomer 268. This is believed to be due to the proximity of the /3-vinyl hydrogen that is a result of both the shorter double bond and its forced coplanarity with the metal. Allene formation from 269 might be induced by blocking the vinyl position (see Sections IV,B and IV,C), but this has not been tested. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

A variety of organometallic, ir complexes are now known, in which both TT-cyclopentadienyl and ir-cycloheptatrienyl ligands are bonded simultaneously to the same transition metal, providing a series of unique mixed sandwich complexes. The first of these, x-cyclopentadienyl-x-cyclo-heptatrienyl-vanadium (XXXI), was described by King and Stone (153)... 

Reactions of anionic metal complexes with halide-bridged dimers can also be used to form a bond between two different metal atoms. Reaetion of (/j -C7H7)Fe(CO)3 with Mn2(CO)gBr2 gives ()j -C7H7)MnFe(CO)6 in which the cycloheptatrienyl ligand coordinates as a diolefin to manganese and as an allyl toward iron . 

Nucleophilic attack on coordinated unsaturated hydrocarbons is one of the fundamental and particularly well studied reactions in Organometallic Chemistry. The addition of carbonylme-tallates instead of common nucleophiles provides a directed synthesis of hydrocarbon bridged complexes. Carbonylmetallates (particularly Re(CO)5", Os(CO)4 ") add to 7C-bonded olefin, acetylene, allyl, diene, trimethylenemethane, dienyl, benzene, triene and cycloheptatrienyl ligands in cationic complexes and give hydrocarbon bridged bi- and trimetallic, homo- or he-teronuclear complexes [1]. 

The structure of arene sandwich compounds (like that of cyclopentadienyl, cycloheptatrienyl, etc., derivatives) and the character of the M-arene bond may also be considered on the basis of ligand field theory assuming C ,v symmetry of the compound (cf. Chapter 9). The ligands connected to the metal via carbon atoms create a strong field. Therefore, the splitting parameters are large. The nephelauxetic coefficients P = BIBq, where B is the Racah parameter for the complex and Bq is the Racah parameter for the free metal ion (5o and B represent interelectron repulsion), generally assume values 0.5 0.1 for metallocenes and arene sandwich complexes. Low values of nephelauxetic coefficients indicate considerable covalent character of the metal-hydrocarbon bond. 

Two heterobimetallic complexes with a Pd-Fe bond supported by a bridging cycloheptatrienyl group have been reported (Scheme 41). In both cases, the cycloheptatrienyl group binds iron as 77 -allyl. For R = Tj -cyclopentadienyl, the complex formed is [ OC)3Fc (p2W 7 -C7H7 Pd( 7 -CsHs)], with Pd-C distances to the two carbons of the... 

The cycloheptatrienyl carbonyl anions [(C7H7)M(CO)3] involve // -coordination of the seven-membered rings at low temperatures (6) even though the // -bonded structure (7) is only slightly less stable.These complexes are fluxional, most probably as a result of a series of 1,2-shifts. Such a mechanism, however, does not require passage through the form (7). 

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