Beryllium , ligand exchange

G. Ligand Exchange on Five Coordinated Beryllium Dications 560... 

Formally, the lone pairs on molecular nitrogen, hydrogen cyanide, and carbon monoxide are sp hybrid orbitals, whereas NLMO hybridizations calculated even lower p contributions. Hence, these lone pairs have low directionality, the electron density remains close to the coordinating atom and interaction between the lone pair and the Be2+ is comparatively weak. The Be-L bonds are easily disrupted and ligand exchange consequently can proceed with a low activation barrier. A high degree of p character, on the other hand, means that the lone pair is directed toward beryllium, with electron density close to the metal center, and thus well suited for coordination. 

NMR has been used to study ligand exchange in non-coordinating solvents for a series of Be(C104)2-4L (L = trimethyl phosphate, DMSO, DMA, DMF, NMA, 1,1,3,3-tetramethylurea, dimethyl methylphosphonate and dimethyl phenylphosphonate) complexes. 3H NMR studies show that the mode of activation for exchange on beryllium varies from dissociative to associative depending upon L and the nature of the non-coordinating solvent.127 128... 

Delpuech et al. (106) examined the tetrahedral solvates of the beryllium cation with trimethyl phosphate (TMPA), dimethyl methyl phosphonate (DMMP), N,iV-dimethylamido-0,0 -dimethyl phosphate (DMADM P), bis-A,iV-dimethylamido-0-methyl phosphate (TMDAMP), and hexamethylphosphorotriamide (HMPT) by NMR including that of Be. The kinetics of ligand exchange ... 

In contrast, no such invariability of mechanism apparently holds for ligand exchange at beryllium(II). Lincoln and Tkaczuk " have used the variable-temperature proton nmr technique to investigate the exchange of... 

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... 

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