THE UNUSUAL FLUXIONAL STRUCTURE OF TETRAMETHYLOXOTUNGSTEN - QUANTUM-CHEMICAL STRUCTURE PREDICTIONS FOR THE D(0) AND D(1) COMPLEXES [MOR4] (M = W, RE, R = H, CH3)

An unprecedented, highly fluxional structure is predicted for the d(0) complex tetramethyloxotungsten, [WO(CH3)(4)] (2), from density functi onal and ab initio calculations: The lowest energy structure 2C(s)-1 o n the potential energy surface may be described equally well as a dist orted trigonal bipyramidal or distorted square pyramidal arrangement. Four equivalent minima 2C(s)-1 may interconvert rapidly via four equiv alent, low-lying transition states 2C(s)-2 (ca. 3 kJ mol(-1) above 2C( s)-1). The more conventional regular square pyramidal structure 2C(4v) -1 is found to have a twofold degenerate imaginary vibrational mode. I t thus also serves as a transition state (only ca. 8 kJ mol(-1) above 2C(s)-1). Interestingly, rotation of the methyl groups is severely hin dered in this square pyramidal structure, whereas it is easy for some of the methyl groups in the distorted structures. Thus, the methyl gro up conformations and the skeletal distortions appear to be strongly co upled due to agostic interactions. Comparisons are made with the model d(0) oxohydride [WOH4] (1), and to the corresponding d(1) complex [Re OH4] (3), which are also fluxional but differ from 2 and from each oth er, The related d(1) complex [ReO(CH3)(4)] (4) prefers the experimenta lly confirmed conventional square pyramidal structure 4C(4v)-1, but is found also to have unusually high barriers for methyl group rotation. Computed IR spectra might aid the experimental structure analysis for tetramethyloxotungsten. Somewhat surprisingly, the predicted O-17 NMR chemical shifts may also provide an indirect test for the structural distortions. The electronic structure characteristics and the competit ion between sigma- and pi-bonding in these interesting compounds are d iscussed, and are related to other d(0) and d(1) species by natural po pulation analysis and frontier orbital arguments. Further low-symmetry structures are suggested.