Tetrahapto ligands

Conjugated dienes such as butadiene and its open-chain analogues can act as r ligands the complexes are usually prepared from metal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur as indicated schematically below  

No new principles are involved in describing the bonding in these complexes and appropriate combinations of the 4p,r orbitals on the diene system can be used to construct MOs with the metal-based orbitals for donation and back donation of eiecuon density. As with ethene, two limiting cases can be envisaged which can be represented schematically as in Fig. 19.25. Consistent with 

Cyciobutadiene complexes are also well established though they must be synthesized by indirect routes since the parent dienes are either unstable or non-existent. Four general routes are available  

A schematic interpretation of the bonding in cyciobutadiene complexes can be given within the framework outlined in the preceding sections and this is illustrated in Fig. 19.26. 

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The crystal structure of (>)" -cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand °. For this complex and similar iron-, ruthenium- and osmium-( i4-cyclooctatetraene)(arene) complexes, their ll and V, NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be /i.6 kcalmol. ... 

A different type of dynamic process may be observed in the compound in which the carbon atoms of the tetrahapto ligands, uncoordinated to the metal, are tilted away from the compound. At 263 K, the NMR spectra are in agreement with the structure (tj -arene) Ru( / -arene), while at 323 K, the methyl groups are equivalent on the NMR time scale. A line-shape analysis of the DNMR spectra showed that there is the following rearrangement Ruirj -C Me Xr/ C Me ) = Ru(f/ -C6Me6)2 = 65.3 5.5 kJ mol ... 

The reduction of [Cr(PhH)2] by means of potassium affords the anion [Cr(PhH)2] in which one benzene molecule acts as an rf ligand while the other is an rf group as evidenced by the ESR data/ The tetrahapto ligand is not planar. The two carbon atoms are tilted away from the complex as in the case of Ru(C6Me6)2. The ESR spectrum confirms that the unpaired electron is located in the n orbital of the ligand. The anisotropy Igy — gj.1, which is higher for this compound than for aromatic radicals, results from the interaction of the tt orbital of the ligand with the orbitals of the chromium center. 

Complexes of platinum with tetrahapto ( j4) cyclobutadiene ligands can be prepared examples are shown in equations (288) and (289).871,872. ... 

Phosphine and phosphite ligands react with arene tricarbonyl complexes of Cr, Mo, and W yielding complexes of the type /ac-(PRg)3M-(CO)3 32, 261, 341-343, 466). Kinetic studies reveal that the reaction proceeds by an 8 2 mechanism 341-343, 466) which is thought to involve the stepwise displacement of the arene via tetrahapto- and dihapto-coordinated arene intermediates 466) ... 

The NMR spectrum showed the presence of a tetrahapto-coordinated (MeOOC)gCg ligand. The rhodium compound showed fluxional NMR behavior on heating to 150°C in dg DMSO which was reversed on cooling. 

NMR behavior down to -90°C where the single triplet resonance (Jpp = 3.0 Hz) became two unresolved multiplets. The low activation energy for the intramolecular rearrangement of the Ni and Ft complexes is in sharp contrast to the compound [(CF3)gCe](7r-C5H5)Rh which has been shown to contain a tetrahapto-coordinated (CF3)eCg ligand (65). 

The i C NMR data for these and other tctraAapto-complexes are given in Table XLVII. As was the case for a-olefinic complexes, the C NMR data for these tetrahapto-comp t ts do not allow any further insight into the nature of metal-ligand bonding. For example, the bonding in a 1,3-diene complex may be viewed in extreme forms as either a bis-olefin complex or a metallo-cyclopentene complex, shown below. The C NMR parameters... 

Liberation of cyclobutadiene from its tetrahapto-iron tricarbonyl complex is believed to proceed via a dihapto iron complex, and the first such complex to be prepared by an independent method is [824 Fp — T -CpFe(CO)2]. On oxidation with iodide ion in the presence of diphenylisobenzofuran the adduct (825) of benzo-cyclobutadiene is obtained. A dinuclear complex with cyclobutadiene acting as a bridging ligand between two iron atoms has also been prepared. ... 

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