First Paramagnetic Zerovalent Transition Metal Isocyanides. Syntheses, Structural Characterizations, and Magnetic Properties of Novel Low-Valent Isocyanide Complexes of Vanadium¹

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. Cs 3 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 [Et 3 NH]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 ][PF 6 ] ( 4 [PF 6 ]), 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 [PF 6 ]. 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 are 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 1 H and 13 C 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 π (M)−p π *(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 π (V)−p π *(CNXyl) back-bonding.