29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency

We report the use of Si-29 NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu(3))(2)(THF)(2)(THF)(x)] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu(2)Me)(2)(THF)(2)(THF)(x)] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and Si-29 NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)(3)}(-)-, {Si(SiMe2H)(3)}(-)-, and {SiPh3}(-)-substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound Si-29 NMR isotropic chemical shifts, delta(Si), span a wide (similar to 225 ppm) range when the metal is kept constant, and direct, linear correlations are found between dSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The dSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) approximate to Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Automated summary

In this study, Si-29 NMR spectroscopy and DFT calculations were combined to investigate the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The results showed a wide range of metal-bound Si-29 NMR isotropic chemical shifts and linear correlations between dSi and computed delocalization indices. The calculations revealed dominant s- and d-orbital character in the bonding of these silanide complexes.