Metal carbene complexes rotational barriers about

Nuclear magnetic resonance (nmr) studies have indicated the partial doublebond character of the carbon-heteroatom bond in the metal-carbene complexes as depicted in resonance structure Ic. The barrier to rotation about the carbon-nitrogen bond in II was found to be greater than 25 Real mole ... 

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

Calculations on [Fe(C5H5)(CO)2(CH2)] (30) (only isolated for substituted carbenes) show that there are suitable metal -orbitals perpendicularly disposed and although the orientation shown is favoured the calculated rotational barrier is only 6.2 kcal mol but this has not been determined experimentally. The car bene complexes [Fe(C5H5)(CO)L(RNHCNHMe)]+ (L=MeNC or tertiary phosphines) undergo intramolecular isomerization but this involves rotation about C—N rather than Fe—C bonds, and the rotational barriers (16.7—17.9 kcal mol ) are essentially independent of L and R. ... 

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