Ethylene Complexes of the Early Transition Metals: Crystal Structures of [HfEt4(C2H4)] and the Negative-Oxidation-State

Alkylation of HfCl4 with 6 equiv of ethyllithium followed by the addition of N,N,N′,N′-tetramethylethylenediamine gives the d hafnium(II) alkyl/alkene complex [Li(tmed)]2[HfEt4(C2H4)]. The X-ray crystal structure of this complex shows that the ethylene ligand has considerable metallacyclopropane character. The C-C distance of 1.49(6) Å and the JCH coupling constant of 119 Hz both support this formulation. A labeling study showed that the ethylene ligand is formed by means of a -hydrogen abstraction process. Similar treatment of TaCl2(OMe)3 and WCl3(OMe)3 with ethyllithium affords the d tantalum(-I) and d tungsten(-II) complexes [Li(tmed)]3[TaHEt(C2H4)3] · /2tmed and [Li(tmed)]3[WH(C2H4)4]. Both trianionic complexes, in which the metal centers have negative oxidation states, adopt distorted-squarepyramidal geometries with the hydride group in the axial position. The C-C bond lengths of 1.484(8), 1.502(8), and 1.515(8) Å for the ethylene groups in the tantalum complex are somewhat longer than those of 1.357(11)–1.466(10) Å in the tungsten complex. In both complexes, the Ta-H and W-H bonds are relatively long at 2.07(9) and 2.04(5) Å, due to the presence of close contacts with two lithium cations. In both the tantalum and tungsten complexes, two of the ethylene ligand(s) are involved in interactions with the lithium cations; these ethylene ligands rotate slowly about their metal–ligand axes on the NMR time scale at -80 °C, whereas at this temperature the remaining ethylene ligands are rotating rapidly. At higher temperatures, the inequivalence of the ethylene environments is maintained in the tantalum complex, but they exchange in the tungsten compound. The activation parameters for the latter process are ∆H ) 9.5(10) kcal mol and ∆S ) –5(4) cal mol K. Spin saturation transfer experiments show that there is no exchange of the W-H and W-C2H4 environments on the NMR time scale. Qualitative molecular orbital descriptions of the bonding in all three compounds are presented.