One electron oxidn. of the beta-Diketiminate vanadium (III) bis-neopentyl complex (Nacnac)V(CH2tBu)2 (Nacnac-=[Ar]NC(Me)CHC(Me)N[Ar], Ar=2,6-(CHMe2)2C6H3) promotes alpha-abstraction to afford the cationic four-coordinate neopentylidene vanadium complex [(Nacnac)V=CHtBu(THF)][BPh4]. The neutral vanadium neopentylidene complex (Nacnac)V=CHtBu(I) was prepd. by the reaction of cationic complex [(Nacnac)V=CHtBu(THF)][BPh4] with I-. This family of alkylidene complexes are one electron paramagnets and display well-resolved EPR spectra at room temp. The thermal stability of these systems was explored in addn. to its reactivity. In conjunction with the above studies, novel four-coordinate vanadium(IV) phosphinidene complexes (Nacnac)V=PR(CH2tBu) (R=2,4,6-iPr3C6H2, 2,4,6-tBuC6H2) were also prepd. from salt metathesis of (Nacnac)V=CHtBu(I) with LiPHR. Solid and soln. magnetic measurements, EPR spectra, and single crystal X-ray diffraction studies have been carried out for all the complexes described. Two electron-oxidn. and double alpha-hydrogen abstraction starting from (Nacnac)V(CH2tBu)2 lead to the first four coordinate neopentylidyne complex (Nacnac)V.tplbond.CtBu(Otf) which showed a distinct alkylidyne carbon resonance at 375 ppm at -50 DegC. [on SciFinder (R)]