WH2Cl2L4 (L = PMe2Ph), formed by oxidative addition of H-2 to WCl2L4, is stereochemically rigid, and, in solution, undergoes exchange with D2 by a mechanism dissociative in L; NMR evidence is consistent with WH2Cl2L4 being a crowded molecule. Because of this mechanism, WH2Cl2L4 is a catalyst for conversion of HD to H-2 and D2. LiDBEt3 converts WH2Cl2L4 into exclusively one isotopomer of WH2FClL4, a dodecahedron which is stereochemically rigid on the time scale of 1 day. The mechanism of this halide substitution also appears to begin with phosphine dissociation. Attempted nucleophilic substitution of one or more chlorides in WH2Cl2L4 by KN(SiMe3)2 or LiR (R = (n)Bu, (t)Bu, CH2tBu, Me, NMe2) leads instead to dehydrohalogenation (followed by ortho-metalation of the P-Ph group, giving a nonfluxional WH2Cl(eta2-C6H4PMe2)L3) or to beta-hydrogen elimination (to give stereochemically-rigid WH3ClL4 or WH3(eta2-C6H4PMe2)L3). The last compound was shown by X-ray diffraction to have a dodecahedral structure of mirror symmetry. Crystal data (-170-degrees-C) : a = 13.770 (7) angstrom, b = 16.567(9) angstrom, c = 14.245(8) angstrom, and beta = 99.97(2)degrees with Z = 4 in space group P2(1)/c. The metalation products are proposed to arise from an intermediate WIIHClL4, WhiCh is lightly stabilized by Cl --> W pi donation.