Crystal structure of trans-[W(CO)4(η2-C2H4)2] and IR and 1H NMR studies of the reactions of this and related ethene carbonyl complexes of tungsten(0), [W(CO)n(η2-C2H4)6- n] ( n=3–5)

Abstract

The structure of trans-[W(CO)₄ (η²-C₂H₄)₂] 1 has been determined by X-ray crystallography at 293 K [orthorhombic, space group Aba2, a=12.458(3), b=6.370(2), c=12.557(3) Å, Z=4, R=0.022] to confirm that the CC bonds of the two ethene ligands are mutually staggered while eclipsing the central W(CO)₄ unit. Reversible isomerisation of trans- to cis-[W(CO)₄(η²-C₂H₄)₂] 2 under the action of selective irradiation has been shown to occur in solid argon matrices at 14–16 K. ¹H NMR measurements have also been applied to the identification of not only 1 and 2 but also the labile 18e complexes [W(CO)₅(η²-C₂H₄)] 3, mer-[W(CO)₃(η²-C₂H₄)₃] 4, and fac-[W(CO)₃(η²-C₂H₄)₃] 5 formed by broad-band UV-visible photolysis of ethene-saturated hydrocarbon solutions of 1 or [W(CO)₆] in the temperature range 200–293 K. In these circumstances the mixed ethene carbonyl complexes 2–5 decay thermally with dissociation of an ethene ligand and substitution by CO in reactions which have been monitored by reference to the relevant ¹H NMR signals. Pseudo-first-order decay constants in the range 0.6–12×10^-4 s^-1 have thus been estimated with temperature-dependences implying activation energies, E_a, that decrease in the order 3>2>4 >5.