Simple Ligand Rotation at a Metal Center

There continue to be reports on rotations of coordinated alkenes, alkynes, etc. about the metal-ligand axis rapid enough to coalesce nmr signals. In square planar complexes the alkene is preferentially orientated perpendicularly. Orientations for other coordination geometries have also been examined experi- 

The alkene ligands in rra -[M(CO)4(methylacrylate)2] (M = Mo or W) are mutually perpendicular and the two diastereomers 3 and 4 are not interconvertible by alkene rotation. Rotational barriers are somewhat higher for W than Mo and are different for the two diastereomers 69.4 2.0 (3 M = W) and 81.5 2.0 kJ mol (4 M = W). This illustrates the dangers in accounting for differences in barriers in simple terms in this case steric effects must be essentially the same in 3 and 4 while electronic differences must be rather subtle. Alkyne rotation is observed in c/5-[W(CO)(Me2NCS2)2(alkyne)] (5) to occur 

Rotation of an ligand is also required in the isomerization of the T] -acyl compound 6 to 7. Considerably slower isomerizations occur when R = aryl, for example, AG (214 K) = 63.5 kJ mol (R = Ph) compared with AG  

The plane of the CH2 ligand in [Fe(C5H5)(Ph2PCH2CH2PPh2)(CH2)] is perpendicular to the FeP2 plane to maximize Fe-C it bonding. The two CH2 hydrogen atoms are therefore nonequivalent ( H nmr signals at 817.29 and 813.89) but there is restricted rotation about the Fe-CH2 bond (7 — 30 C). 

Compared with [Fe(C5H5)(CO)2(CH2)] the diphosphine complex would have greater multiple iron-carbon bonding giving a greater rotational barrier and reduced electrophilicity of the CH2 ligand. 

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