TETRACYANOETHYLENE ANION AS A HARD SIGMA BASE . SYNTHESIS AND STRUCTURE OF 1-1 ADDUCT OF TETRACYANOETHYLENE AND BIS(PI-CYCLOPENTADIENYL)VANADIUM MONOBROMIDE

Reaction of (π-C5Hs)2VX (X = Cl, Br, I) with tetracyanoethylene (TCNE) gives rise to the 1:1 adducts (π-C5H5)2VX.TCNE. The infrared spectra of the adducts are nearly identical and are the same in both the solid and solution phases. A single-crystal X-ray study of the bromo adduct shows that TCNE has added to vanadium through a nitrile nitrogen, rather than through the π-olefinic bond. Thus, the coordination about vanadium is distorted tetrahedral, and the mode of TCNE attachment is different from all previously known transition metal-TCNE complexes. The adducts are best represented as formal vanadium(IV) complexes of tetracyanoethylene anion. This conclusion is reached on the basis of the low C≡N and C=C stretching frequencies. The details of the molecular structure suggest an intramolecular Br--TCNE interaction. The observed structureis consistent with ahard acid ((π-C5H.)2VX+)-hard base (TCNE-) interaction, in contrast to the previous structures of TCNE complexes involving soft acid (TCNE)-soft base (metal complexes) interactions. The crystals studied belonged to space group Pccn with a0 = 21.09 (4), b0 = 11.68 (2), and C0 = 12.79 (2) Å. The observed density was 1.64 g/cm3, giving a calculated value of 8.01 formula units in the cell. The structure was solved using 577 visually estimated unique reflections and refined to a final R factor of 13.0% based on F. © 1969, American Chemical Society. All rights reserved.