Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [W-IV(QR)-(S2C2R2')(2)](1-) and [(WO)-O-VI(QR)(S2C2R2')(2)](1-) (Q = O, S, Se) as minimal representations of enzyme sites. The existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)(2)(S2C2Me2)(2)] (1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)(2)](1-) (2). Reaction of 1 with PhQ(-) affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)(2)](1-) (Q = S (3), Se (5)). The use of sterically demanding 2,4,6-Pr(3)(i)C(6)H(2)Q(-) also yields monocarbonyls, [W(CO)(QC(6)H(2)-2,4,6-Pr-3(i))(S2C2Me2)(2)](1-) (Q = S (4), Se (6)). The X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described. The tendency to substitute one or both carbonyl ligands in 1 in the formation of [M-IV(QAr)(S2C2Me2)(2)](1-) and [M-IV(CO)(QAr)(S2C2Me2)(2)](1-) with M = Mo and W is related to the M-Q bond length and ligand steric demands. The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2 (-2.07 V), monocarbonyl ligation stabilizes W(III) by similar to 500 mV. This work is part of a parallel investigation of the chemistry of bis-(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten complexes related to enzyme active sites.