The nucleophilic reactivity of oxo ligands in the groups (MO3)-O-VI in the trigonal complexes [(Me(3)tacn)MO3] (M = Mo (1), W (10)) and [(Bu(3)(t)tach)MO3] (M = Mo (5), W (14)) has been investigated. Complexes 1/10 can be alkylated with MeOTf to give [(Me(3)tacn)MO2(OMe)(1+) (2/11), silylated with (Pr3SiOTf)-Si-i to form [(Me3tacn)MO2(OSiPr 3)](+) (3/12), and protonated with HOTf to yield [(Me(3)tacn)MoO2(OH)](+) (4). Similarly, complexes 5/14 can be silylated to [(Bu(3)(t)tach)MO2(OSiPr3i)](+) (6/15) and protonated to [(Bu(3)(t)tach)MO2(OH)](+) (7/16). Products were isolated as triflate salts in yields exceeding 70%. When excess acid was used, the dinuclear mu-oxo species [(Bu(3)(t)tach)(2)M2O5](2+) (8/17) were obtained. X-ray structures are reported for 2-4, 6-8, 12, and 15-17. All mononuclear complexes have dominant trigonal symmetry with a rhombic distortion owing to a M-OR bond (R = Me, SiPr3i, H), which is longer than M=O oxo interactions; the latter exert a substantial trans influence on M-N bond lengths. Oxo ligands in 5/14 undergo replacement with sulfide. Lawesson's reagent effects formation of [(Bu(3)(t)tach)MS3] (9/18), 14 with excess B2S3 yields incompletely substituted [(Bu(3)(t)tach)WOS2] (20), and 5 with excess B2S3 yields [(Bu(3)(t)tach)(MoO)-O-IV(S-4)] (19). The structures of 9, 19, and 20 are reported. Precedents for (MS3)-S-VI groups in five- and six-coordinate molecules are limited. This investigation is the first detailed study of the behavior of (MO3)-O-VI groups in nucleophilic and oxo/ sulfido substitution reactions and should be useful in synthetic approaches to the active sites of the xanthine oxidase enzyme family and of certain tungstoenzymes. (Bu(3)(t)tach = 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane, Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclonane; OTf = triflate).