Rotations about the Metal-Ligand Bond

Ligand Rotations about the Metal-Ligand Bond 

Commencing with alkyl group rotations, there have been two studies of the stereodynamics of chromium carbonyl complexes with 77 -hexaalkylbenzene. For the complex [ Cr(CO)2L 2(/i-N2)] (L = ry -hexaethylbenzene), NMR band-shape changes were attributed to slowed ethyl group rotation with a barrier, AG (300 K), of 46.0 3.0 kJ moP. The ligand hexa-n-propyl-benzene adopts a D3d symmetry with the alkyl substituent alternately up and down with respect to the observer. This geometry is retained in the chromium tricarbonyl complex and a decoalescence phenomenon observed in its NMR spectrum was 

Olefin rotation occurs in the complexes mer-tricarbonyl(T7 -olefin)(T7 -norbor-nadiene) tungsten (3) with Arrhenius activation energies of 58 and 54.8 kJ moP for complexes, L = T/ -(Z)-cyclo-octene and 17 -ethene, respectively. Restricted 

Monoalkyne tungsten(II) complexes of type [W(LL )2(CO)(RC=CR )] (LL = S2CNMe2, S2PMe2, etc.) (5) display fluxional behavior due to propeller rotation of the coordinated alkyne when R = R = Me, LL = S2CNMe2 or 2-SC5H4N, but when R = Ph, R = Me, Ph, and LL = S2PMe2 an alternative process 

A number of NMR studies have been directed toward restricted rotations of substituted cyclopentadienyl ligands in metallocenes. For instance, in octa-isopropyl ferrocene the activation barrier for tetraisopropyl cyclopentadienyl 

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Ligand Rotations about the Metal-Ligand Bond. 341... 

H NMR shows that the alkyne ligand in 242 (R = H) rotates about the metal-alkyne bond axis with an activation barrier of only 50 kJ/mol. The structure of the diphenylacetylene complex 242 was determined by X-ray crystallography. The reaction shown in Eq. (183) was expected to generate alkylidyne alkyne complexes such as W(CPh)CI(PhC2R)(CO)-(PMe3)2. Such species are probably intermediates in the reaction however, a n conflict between the adjacent C=C and M=C triple bond systems... 

The NMR of rrring carbons and eleven resonances were observed for the A -/ro/ij-l,3-dimethylindane ligand. Only a single carbonyl resonance was observed, 235.0 ppm, even down to — 80°C which suggests that the barrier to rotation about the metal-arene bond is quite low even in this relatively crowded complex. Alternatively the Cr(CO)3 moiety may be non rigid. 

In (Ti -Cp)Rh(Ti -C2H4)2, two alkene proton signals are observed at 233 K (the dilferent H environments are red and black respectively in 24.55). At 373 K, the proton environments become equivalent on the NMR spectroscopic timescale as each alkene ligand rotates about the metal-ligand coordinate bond. 

A coordinated olefin is often most stable in one orientation, relative to the other ligands at the metal. In many cases, the barrier to rotation about the metal-olefin bond axis can be measured by solution NMR methods. The rotational barriers (10-25 kcal/mol) arise from a combination of steric and electronic effects. 

In this complex, the molecule of coordinated ethylene occupies a single coordination site and is oriented perpendicularly to the plane of the square anion. In both complexes mentioned above (i.e., the complexes with two-center ligands and the olefin rr complexes), the bonding of ligands to the central atom occurs via the dihapto type. Moreover, the rotation of olefin about the metal-ligand bond was observed both in Zeise s salt and in some other ir complexes. Thus, the free energies for the activation of olefin (L) rotation in complexes (PR3)PtCl2 -L and PtCl(acac) - L depend on the nature of R and L, and vary within 44.4-67 kJ mol (10.6 16 kcal mol ) [80]. 

Chromium.— The acetylene ligand in [(A -C5H6)Cr(NO)(CO)(C2H2)] adopts a preferred conformation at low temperatures ( < 223 K). Above ambient temperature, rotation about the metal-alkyne bond takes place and the two low-temperature n.m.r. signals coalesce. The free activation energy (51.6—... 

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