Metal complexes of heteroarenes. 11. eta(6)-coordination of arsenine to titanium, vanadium, and chromium

By means of metal-ligand vapor co-condensation techniques the homoleptic ar senine sandwich complexes (eta(6)-C5H5As)(2)Ti (2), (eta(6)-C5H5As)(2)V (3) , and (eta(6)-C5H5As)(2)Cr (4) have been prepared. 2 is the first example o f an unsubstituted group 15 heteroarene sandwich complex which yielded to f ull structural characterization by X-ray diffraction. The most remarkable f eature of the structure of 2 in the crystal is the synperiplanar conformati on and the short intramolecular interannular As As distances which imply se condary bonding. The latter may also contribute to the packing since intera nnular As As distances, which fall short of the sum of the van der Waals ra dii, are detected. The result of a competition experiment, in which benzene and arsenine are offered as ligands to chromium, illustrate the pronounced preference of chromium for arsenine as an eta(6) ligand. The mixed-ligand complexes (eta(6)-C5H5As)(eta(6)-C6H6)Cr (6) and (eta(6)-C5H5As)Cr(CO)(3) ( 8) have also been prepared and studied spectroscopically in order to underp in the notion that arsenine, compared to benzene, is the superior eta(6) li gand. The study is rounded off by an investigation of the redox behavior of 2, 3, 4, and 6 (cyclic voltammetry) and by EPR measurements on 3(.) and 4( .+). The latter confirm the pronounced pi-acceptor character of eta(6)-arse nince. This conclusion is based on the increase of the hyperfine coupling c onstant a(V-51) upon going from bis(benzene)vanadium to the arsenine counte rpart which is thought to reflect V(d(z)2) orbital contraction in 3(.), cau sed by a slight increase of positive partial charge on vanadium.