Symmetry of Metal Chelates

Is a metal chelate symmetric, with the motion of the metal described by a single-well potential, or is it asymmetric,
in a double-well potential? For hydrogen, this is the familiar question of the symmetry of a hydrogen bond. The
molecular symmetry of MLn complexes (M = Li, Na, K, Al, Pd, Rh, Si, Sn, Ge, Sb, etc.; L is the anion of
3-hydroxy-2-phenylpropenal) in solution is now probed with the method of isotopic perturbation of equilibrium.
A statistical mixture of 3-hydroxy-2-phenylpropenal-d0, -1-d, and -1,3-d2 was synthesized and converted to various
metal complexes. Some complexes show two aldehydic signals, which means that their ligands are monodentate.
For LiL, NaL, and KL, the 13C NMR isotope shifts, δCH(D) − δCH(H), for the aldehydic CH groups are small and
negative, consistent with L- being a resonance hybrid. They are small and positive for AlL3, PdL2, Rh(CO)2L,
SiX3L, SiL3+X-, (CF3)3GeL, SbCl4L, (EtO)4TaL, and (EtO)4NbL. The positive isotope shifts are unusual, but
since they are small and temperature independent, they are intrinsic and indicate that these metal chelates are
symmetric, as expected. Large positive isotope shifts, up to 400 ppb, are observed for Ph3GeL, Me3GeL, Ph2GeL2, Bu3SnL, and Ph4SbL. However, it is likely that these are monodentate complexes undergoing rapid metal
migration, as judged from the X-ray crystal structures of Ph3SnL and Ph4SbL. NMR experiments indicate an
intermolecular mechanism for exchange, which may be a bimolecular double metal transfer. It is remarkable that
the isotope shifts in these five complexes demonstrate that they are asymmetric structures, even though they
appear from other NMR evidence to be symmetric chelates.