First paramagnetic zerovalent transition metal isocyanides. Syntheses, structural characterizations, and magnetic properties of novel low-valent isocyanide complexes of vanadium

The first homoleptic paramagnetic transition metal isocyanide, V(CNXyl)(6) (2, Xyl = 2,6-dimethylphenyl), can be isolated in high yield by reacting bis(naphthalene)vanadium(0) (1a) or bis(1-methylnaphthalene)vanadium(0) (1b) with 6 equiv of CNXyl in tetrahydrofuran/heptane. Reduction of 2 with excess cesium graphite in THF affords excellent yields of [V(CNXyl)(6)](-) (3) as an unsolvated Cs+ salt, the first homoleptic octahedral isocyanide metalate. Cs3 reacts with 2 equiv of 18-Crown-6 to give [Cs(18-Crown-6)(2)]3. Anion 3 can also be isolated as a practically insoluble [K(Crypt{2.2.2.})](+) salt by reducing 2 with potassium naphthalenide in the presence of Crypt{2.2.2.}. Complex 3 reduces [Et3NH]Cl to form paramagetic 2. Oxidation of 2 by ferricinium hexafluorophosphate in THF provides nearly quantitative yields of the 16-electron paramagnetic [V(CNXyl)(6)][PF6] (4[PF6]), analogous to the exceedingly unstable [V(CO)(6)](+), interaction of V(CO)(6), with excess CNXyl in heptane results in the efficient formation of trans-[V(CO)(2)(CNXyl)(4)] (5). Such a substitution reaction is highly unusual for V(CO)(6). Oxidation of compound 5 by ferricinium hexafluorophosphate in THF affords homoleptic 4[PF6]. Complexes 2, 3, 4, and 5 were characterized by a variety of spectroscopic methods and X-ray crystallography. Spectroscopic, magnetic, and structural features of these novel electron rich vanadium isocyanides ate discussed in detail. The average V-CN bond length increases in the series [V(CNXyl)(6)](-) < V(CNXyl)(6) < trans-V(CO)(2)(CNXyl)(4) < [V(CNXyl)6](+). Well-resolved H-1 and C-13 NMR spectra were obtained for paramagnetic 2 and its chromium congener, [Cr(CNXyl)(6)](+). The importance of back-bonding in the mechanism of unpaired spin delocalization within 2 was demonstrated. Contrary to the previous prediction for d pi(M)-p pi*(L) unpaired spin delocalization in low-spin d(5) octahedral complexes, negative spin appears to be induced on the CNXyl ligands of 2 by means of d pi(V)-p pi*(CNXyl) back-bonding.