Metal-diene complex

The crystal structures of a number of s-trans (diene)metal complexes (3) have been determined10-14. The diene ligand in all s-trans complexes is distinctly non-planar the torsional angle between the two olefin groups is between 114° and 127°. In general, the terminal carbon to metal distance is greater than for the internal carbon to metal distance, and the Cl—C2/C3—C4 bonds are shorter than the C2—C3 bond. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

By far the greatest number of diene-metal complexes are of the type (diene)Fe(CO)3. All of these complexes exhibit the s-cis rf -diene coordination mode. 

A number of cationic (diene)metal complexes undergo a-deprotonation. Treatment of (cyclodiene)Mo(CO)2Cp+ cations (e.g. 44) with NEt3 or other non-nucleophilic bases... 

In general, reagents approach a (diene)metal complex on the face opposite to the metal due to the steric bulk of the attached metal-ligand array. Due to the relatively low cost of iron, the vast majority of examples of these type of reactions utilize the Fe(CO)3 fragment. 

II. The Unique (,v-tra ,v-q4-Conjugatcd Diene)Metal Complex Family. Ill... 

THE UNIQUE (s-frans-r 4-CONJUGATED DIENE)METAL COMPLEX FAMILY... 

It is well known that x-traw-butadiene can serve as a bridging ligand in binuclear [( i-r 2,r 2-C4H6)M2L ]metal complexes.12 However, since the original discovery in 19804 there has been an increasing number of structurally, chemically, and spectroscopically well characterized mononuclear (s-trura-conjugated diene)metal complex systems reported in the literature.11... 

Higher 43-hydrocarbyl derivatives of transition metals can be obtained by partial protolysis of the bis(7i-allyl)metal compounds formed from cyclo-octa-1,5-diene metal complexes and 1,3-butadiene with one equivalent of the Bronsted acid in the presence of the proper ligand in a polar solvent at lowered temperature (e.g. — 40°C). For instance, Ni [43-(CxHn)l and Ni—[43-(Ci2Hig)] derivatives have been yielded by the reaction of Ni(CxH 2)2 [(Ni(Cod)2] with 1,3-butadiene (C4H6), followed by treatment with a Bronsted acid, according to scheme (1) [132] and scheme (2) [37] respectively ... 

The 1,3-diene-metal complexes show a number of characteristic features and will be treated separately. Data for selected compounds, all containing single-c/s-butadiene, are collected in Table VII (50, 53 -60). Early studies (61) concentrated on differentiating between the two valence bond representations 16 and 17, while recent interest has centered on intramolecular rearrangements (62). 

Some years ago we observed that Group IV metallocenes, i.e., the elusive monomeric units bis(i7-cyclopentadienyl)zirconium and -hafnium, readily form 1 1 complexes with conjugated dienes in their 5-trans conformation (22). To our knowledge, these complexes (3) are the first and, at the time, only known examples of mononuclear (i-/ranj-i7 -diene)metal complexes, i.e., compounds in which both double bonds of the i-irans conformer of a conjugated diene are coordinated to the same transition metal center 21). Some examples of this class of transition metal complexes are stable, isolable compounds even at room temperature 23). 

Another early discovery relating to diene-metal complexes was described by Reihlen and co-workers in 1930 (210). These workers treated... 

The recent excellent monograph by Fischer and Werner 100) contains a comprehensive coverage of diene-metal complexes up to 1965. Earlier reviews on this topic were published in 1962-1963 by Bennett 11), Guy and Shaw 113), Cais 24), and Fischer and Werner 99). Reviews dealing specifically with the chemistry of diene-iron carbonyl complexes have recently been prepared by Pettit et al. 191, 192). 

TABLE 2. H nmr spectral data (chemical shift S in ppm coupling constants J in Hz) for (s-tranj-diene)metal complexes... 

Except for MCp complexes, most (diene)metal complexes consistently exhibit particular molecular orientations in the solid state. While this is the case, rotation about the metal-ligand axis may be rapid in the solution phase. For example, the crystal structures of a variety of cyclic and acyclic (diene)Fe(CO)3 complexes indicate a staggered geometry (7)2,42. carbonyl ligands is oriented such that it bisects the open end of the... 

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