The halo tricarbonyl complexes LWX(CO)(3) (X = I, Br, Cl) are formed when NEt(4)[LW(CO)(3)] is reacted with I(2), N-bromosuccinimide, and N-chlorosuccinimide (or PhICl(2)), respectively, while reaction of NEt(4)[L(Pr)W(CO)(3)] and I(2) yields L(Pr)WI(CO)(3) [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate, L(Pr) = hydrotris(3-isopropylpyrazol-1-yl)borate]. Reaction of NEt(4)[LW(CO)(3)] with Br(2) yields mixtures of LWBr(CO)(3) and L(Br)WBr(CO)(3) [L(Br) = hydrotris(4-bromo-3,5-dimethylpyrazol-1-yl)borate] as a result of competitive bromination of both tungsten and ligand L. The complexes generally exhibit three nu(CO) bands, one at ca. 2025 cm(-)(1) and two between 1930 and 1900 cm(-)(1), and NMR spectra consistent with fluxional carbonyl-capped octahedral (C(s)()) structures. Crystals of L(Pr)WI(CO)(3).MeOH are orthorhombic, space group Pbca, with a = 11.779(2) Å, b = 15.975(4) Å, c = 29.476(3) Å, and V = 5547(2) Å(3) for Z = 8. The seven-coordinate complex exhibits a 3:3:1 carbonyl-capped octahedral structure. In hot acetonitrile or tetrahydrofuran, the tricarbonyl complexes are converted into brown, paramagnetic (&mgr;(eff) ca. 1.15 &mgr;(B)) dicarbonyl species, LWX(CO)(2) and L(Br)WBr(CO)(2), which exhibit two nu(CO) bands (ca. 1930 and 1840 cm(-)(1)) and isotropically shifted NMR spectra consistent with a mononuclear structure with C(s)() symmetry. Further decarbonylation of L(Br)WBr(CO)(2) in refluxing acetonitrile results in the formation of L(Br)WBr(MeCN-kappa(2)N,C)(CO), which crystallizes in monoclinic space group C2/c, with a = 22.245(4) Å, b = 19.077(4) Å, c = 15.639(3) Å, beta = 128.61(2) degrees, and V = 5286(2) Å(3) for Z = 8. The seven-coordinate complex features a "side-on" bonded, four-electron-donor acetonitrile ligand and a completely 4-brominated L(Br) ligand.