Hydrogen-halogen exchange reactions of FvW 2 (CO) 4 (PPh 2 Me) 2 H 2 (1) with CCl 4 and CHI 3 gave FvW 2 (CO) 4 (PPh 2 Me) 2 Cl 2 (2) and FvW 2 (CO) 4 (PPh 2 Me) 2 I 2 (3), respectively, in nearly quantitative isolated yield (Fv=η 5 :η 5 -C 1 0 H 8 , fulvalene). Both reactions were shown by 1 H- and 3 1 P{ 1 H}-NMR spectroscopy to proceed via FvW 2 (CO) 4 (PPh 2 Me) 2 HX (X=Cl, I) intermediates. Two isomers of the hydrido-halide complexes, containing a cis or trans halide half, were identified in the reaction mixtures. When the reactions were complete, 3 was found to maintain an equilibrium mixture of cis,cis (d,l-/meso-=1:1), cis,trans and trans,trans isomers in solution. The equilibrium was strongly solvent dependent. While minor amounts of the cis,trans (but no trans,trans) isomer were also detected during the formation of 2, this compound was found to exist solely as a 1:1 mixture of d,l- and meso-cis,cis isomers in equilibrated solutions. Crystallization of 3 gave the meso-cis,cis isomer, which was characterized by X-ray diffraction.