Electronic structure comparisons of isostructural early d- and f-block metal(III) bis(cyclopentadienyl) silanide complexes

We report the synthesis of the U(III) bis(cyclopentadienyl) hypersilanide complex [U(Cp '')(2){Si(SiMe3)(3)}] (Cp '' = {C5H3(SiMe3)(2)-1,3}), together with isostructural lanthanide and group 4 M(III) homologues, in order to meaningfully compare metal-silicon bonding between early d- and f-block metals. All complexes were characterised by a combination of NMR, EPR, UV-vis-NIR and ATR-IR spectroscopies, single crystal X-ray diffraction, SQUID magnetometry, elemental analysis and ab initio calculations. We find that for the [M(Cp '')(2){Si(SiMe3)(3)}] (M = Ti, Zr, La, Ce, Nd, U) series the unique anisotropy axis is conserved tangential to Cp ''(cent) - M - Cp ''(cent); this is governed by the hypersilanide ligand for the d-block complexes to give easy plane anisotropy, whereas the easy axis is fixed by the two Cp '' ligands in f-block congeners. This divergence is attributed to hypersilanide acting as a strong sigma-donor and weak pi-acceptor with the d-block metals, whilst f-block metals show predominantly electrostatic bonding with weaker pi-components. We make qualitative comparisons on the strength of covalency to derive the ordering Zr > Ti >> U > Nd approximate to Ce approximate to La in these complexes, using a combination of analytical techniques. The greater covalency of 5f(3) U(iii) vs. 4f(3) Nd(iii) is found by comparison of their EPR and electronic absorption spectra and magnetic measurements, with calculations indicating that uranium 5f orbitals have weak pi-bonding interactions with both the silanide and Cp '' ligands, in addition to weak delta-antibonding with Cp ''.

Automated summary

In this study, the synthesis of U(III) bis(cyclopentadienyl) hypersilanide complex was reported, and isostructural lanthanide and group 4 M(III) homologues were synthesized for comparison of metal-silicon bonding between early d- and f-block metals. The complexes were characterized using various spectroscopic techniques, crystallography, magnetometry, elemental analysis and ab initio calculations. The results showed that the anisotropy axis in the d-block complexes is governed by the hypersilanide ligand, while in the f-block congeners it is fixed by the two Cp '' ligands. It was also found that the covalency in these complexes follows the order Zr > Ti >> U > Nd ≈ Ce ≈ La, with uranium showing greater covalency compared to neodymium. The bonding interactions were analyzed using EPR, electronic absorption spectra, magnetic measurements and calculations.