Vanadium oxo-alkoxide complexes [V(O)(Cl)(3-x)(OR)x] (x = 1, R = Me (1), Et (2), n-Pr (3); x = 2, R = Me (4), Et (5), n-Pr (6)) were obtained by a clean reaction between [V(O)Cl3] and different excesses of ROSiMe3 silicon ethers. Imido complexes [V(NAr)(Cl)(3-x)(OR)x]2 (Ar = 2,6-i-Pr2C6H3; x = 1, R = Me (7), Et (8), n-Pr (9); x = 2, R = Me (10), Et (11), n-Pr (12)) were prepared in excellent yields by reaction of the oxo-alkoxide precursors and ArNCO. X-Ray crystal structure determinations for 7, 8 and 12 revealed dimeric structures with alkoxide bridges. Complexes 7-12 are precursor of highly active guanylation catalysts. The study of the catalytic reaction allow us to propose the non-participation of the imido moiety. This was confirmed when complexes 1-6 gave better catalysts under the same conditions. The simple and commercial [V(O)Cl3] (16) can be proposed as the most effective catalyst in this series of vanadium complexes.