VANADIUM(II) SALTS IN PYRIDINE AND ACETONITRILE SOLVENTS

Earlier work on the interaction of V(II ) salts with pyridine and acet onitrile as solvents has been extended. The structure of the compound VPy(4)(O3SCF3)(2), 1, an intermediate in many of the preparations, has been determined by X-ray diffraction. Analogous solids with I-, Br-, Cl-, SCN-, N-3(-), PhS(-), EtS(-), and BH4- as counterions have been p repared. Attempts to prepare solids with PhO(-), HO-, MeO(-), Ph(-), C N-, AlH4-, and H- failed, but the tetrapyridine complexes were prepare d by titrating a solution of 1 in pyridine with the lithium salt of ea ch anion (in the case of H-, the anion was Et(3)BH(-)) to an end point observed at the 1:2 ratio. Comparisons in a number of cases of the ab sorption spectra and of the cyclovoltammetric behavior of the tetrapyr idine salts in pyridine and in CH2Cl2 show the species in the two solv ents to be the same and to correspond to the composition of the solids . Intercomparisons of the absorption spectra in pyridine suggest that the complexes with the aforementioned anions all have similar structur es: four pyridine molecules situated equatorially-on the basis of crys tal structure determinations, in a propeller arrangement--and the anio ns occupying axial positions. The initial absorption spectrum of a sol ution of VPy(4)(PF6)(2) in pyridine changes to that shown by a solutio n of VPy(6)(PF6)(2) or of VPy(6)(BPh(4))(2), and we conclude that with these weakly nucleophilic anions VPy(6)(2+) is the dominant form of V (II) in pyridine solution. In every case, the prominent feature of the absorption spectrum is a band envelope consisting of a maximum that r anges from 586 to 400 nm accompanied by a shoulder which usually lies on ther low-energy side of the dominant peak and which in these cases we assign to nu(CT) and nu(1)'', respectively. The latter is the highe r energy component of the two transitions nu(1)' and nu(1)'', which ar ise from nu(1) (octahedral) when the symmetry is reduced to axial. Ene rgy correlations suggest that nu(CT) always lies at higher energy than nu(1)'', so that when the shoulder lies on the high-energy side of th e dominant peak, epsilon(nu(1)'') exceeds epsilon(nu(CT)) In a number of cases nu(1)' is observed. It is of much lower intensity than nu(1)' ', and the wavelengths of the transitions lie in the range of 600 nm a nd higher and vary with the nature of ligands as observed in other cas es of axial symmetry. The absorption characteristics of Vqy(6)(2+) res emble those of the tetrapyridine complexes, suggesting that the propel lerlike arrangement is retained in this species, two pyridines replaci ng the anions in the axial positions; i.e., the axial and equatorial p yridines are inequivalent.