Nuclear magnetic resonance metal substitution

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

It must be recognized that precise information about the local structure of metal centers in different phases (solution, glassy or frozen solutions, amorphous) has also been provided by spectroscopic techniques such as nuclear magnetic resonance. X-ray absorption spectroscopy, and electron-nuclear double resonance on native, reduced, or metal-substituted SOD derivatives. 

Relatively few rate and mechanistic studies on tetrahedral complexes have been reported. These complexes are less common than octahedral complexes and their substitution reactions are frequently so fast that their study requires techniques such as stopped flow, temperature jump, or nuclear magnetic resonance. That the reactions are fast indicates that associative processes are occurring, since the activation energy for a reaction will be reduced when an incoming group can assist in the cleavage of a metal—ligand bond. 

It may be noted that the nuclear magnetic resonance contact shifts of some paramagnetic metallocenes and l,l -dimethyl derivatives may not be understood by analogy with those in substituted benzene anions [85c]. Perhaps the mechanism of delocalization of unpaired spin density to the ring substituents of metallocenes involves some direct metal-substituent interaction. 

---