Peralkylated ytterbium(II) aluminate complexes YbAl2R8. New insights into the nature of aluminate coordination

The homoleptic ytterbium(II) tetraalkylaluminate complexes {Yb[AlR4](2)}(n) have been obtained according to a silylamide elimination reaction from Yb[ N(SiMe3)(2)](2)(THF)(2) and excess AlR3 (R = Me, Et, iBu). While the tetram ethylaluminate derivative is a pyrophoric powder which is insoluble in alip hatic and aromatic hydrocarbons, the ethyl and isobutyl congeners are readi ly soluble in n-hexane. Perethylated polymeric {Yb[AlEt4](2)}(n) is constit uted formally of the two molecular fragments [Yb(AlEt4)](+) and [Yb(AlEt4)( 3)](-), forming an intricate three-dimensional network in the solid state. Both fragments are linked by bridging alpha -carbon atoms and secondary Yb . . .H-C agostic interactions combining mu,eta (1), mu,eta (2), and mu,eta (3) coordination modes which result in remarkably short Yb . . . Al (2.809( 2) Angstrom) and a large range of Yb . . .C (2.649(5)-3.364(6) Angstrom) di stances. DFT calculations on the molecular fragments [Yb(AlEt4)(3)](-) and [Yb(AlEt4)](+) reproduced the X-ray geometry remarkably well. Moreover, the theoretical investigations on model systems for the aluminate coordination support the highly fluxional nature of the aluminate coordination (DeltaE( eta2 --> eta3) = -8 kcal/mol), which is also indicated by solution NMR spec troscopy. A topological analysis of the total electron density of the mu,et a (2)-bonded aluminate ligand in the benchmark systems Y(AlR4)(3) (R = Me, Et) revealed the presence of two bond critical points between the Y-C-b and C-b-Al bonds (C-b = bridging carbon atom) and thus suggests a hypervalent character of the bridging carbon atom.