Two different monoanionic O,N-chelating ligand systems, i.e., [OC6H2(CH2NMe2)-2-Me-2-4,6](-) (1) and [OCMe2([2]-Py)](-) (2), have been applied in the synthesis of vanadium(V) complexes. The tertiary amine functionality in 1 caused reduction of the vanadium nucleus to the 4+ oxidation state with either [VOCl3], [V(=NR)Cl-3], or [V(=NR)(NEt2)(3)] (R = Ph, (3a, 5a), R = p-Tol (3b, 5b)), and applying 1 as a reducing agent resulted in the synthesis of the vanadium(IV) complexes [VO(OC6H2(CH2NMe2)-2-Me-2-4,6)(2)] (3) and [V(=NPh)(OC6H2(CH2NMe2)-2-Me-2-4,6)(2)] (6). In the case of [V(=N-p-Tol)(NEt2)(OC6H2(CH2NMe2)-2-Me-2-4,6)(2)] (7b), the reduction was sufficiently slow to allow its characterization by H-1 NMR and variable-temperature studies showed it to be a five-coordinate species in solution. Although the reaction of I with IV(=N-p-Tol)(O-i-Pr)(3)] (9b) did not result in reduction of the vanadium nucleus, vanadium(V) compounds could not be isolated. Mixtures of the vanadium(V) (mono)phenolate, IV (=N-p-Tol)(O-i-Pr)(2)(OC6H2(CH2NMe2)-2-Me-2-4,6)] (10), and the vanadium(V) (bis)phenolate, [V(=N-p-Tol)(O-i-Pr)(OC6H2((CHNMe2)-N-2)-2-Me-2-4,6)(2)] (11), were obtained. With the pyridylalkoxide 2, no reduction was observed and the vanadium(II) compounds [VOCl2(OCMe2([2]-Py))] (12) and [V(=N-pTol)Cl-2(OCMe2([2]-Py)] (13) were obtained. V-51 NMR showed 7b and 12 to be five-coordinate in solution, whereas for 10, 11, and 13 a coordination number of 6 was found. Compounds 12 and 13 showed decreased activity compared to their nonchelated vanadium(V) analogues when applied as catalysts in ethene polymerization. Two polymorphic forms with a difference in the V-N-C angle of 12.5 degrees have been found for 6. Crystal data: 6.Et2O, triclinic, P (1) over bar, a = 11.1557(6) Angstrom, b = 12.5744(12) Angstrom, c = 13.1051(14) Angstrom, alpha = 64.244(8)degrees, beta = 70.472(7)degrees, gamma = 87.950(6)degrees, V = 1547(3) Angstrom(3), Z = 2; 6.C6H6, triclinic, P (1) over bar, a = 8.6034(3) Angstrom, b = 13.3614(4) Angstrom, c = 15.1044(5) Angstrom, alpha = 98.182(3)degrees, beta = 105.618(2)degrees, gamma = 107.130(2)degrees, V = 1551.00(10) Angstrom(3), Z = 2; 12, orthorhombic, Pbca, a = 11.8576(12) Angstrom, b = 12.6710(13) Angstrom, c = 14.722(2) Angstrom, V = 2211.9(4) Angstrom(3), Z = 8.