We have used Fe-57 nuclear resonance vibrational spectroscopy (NRVS) to study the iron site in the iron-sulfur cluster-free hydrogenase Hmd from the methanogenic archaeon Methanothermobacter marburgensis. The spectra have been interpreted by comparison with a cis-(CO)(2)-ligated Fe model compound, Fe(S2C2H4)(CO)(2)(PMe3)(2), as well as by normal mode simulations of plausible active site structures. For this model complex, normal mode analyses both from an optimized Urey-Bradley force field and from complementary density functional theory (DFT) calculations produced consistent results. For Hmd, previous IR spectroscopic studies found strong CO stretching modes at 1944 and 2011 cm(-1), interpreted as evidence for cis-Fe(CO)(2) ligation. The NRVS data provide further insight into the dynamics of the Fe site, revealing Fe-CO stretch and Fe-CO bend modes at 494, 562, 590, and 648 cm-1, consistent with the proposed cis-Fe(CO)2 ligation. The NRVS also reveals a band assigned to Fe-S stretching motion at similar to 311 cm(-1) and another reproducible feature at similar to 380 cm(-1). The Fe-57 partial vibrational densities of states (PVDOS) for Hmd can be reasonably well simulated by a normal mode analysis based on a Urey-Bradley force field for a five-coordinate cis-(CO)(2)-ligated Fe site with additional cysteine, water, and pyridone cofactor ligands. A "truncated" model without a water ligand can also be used to match the NRVS data. A final interpretation of the Hmd NRVS data, including DFT analysis, awaits a three-dimensional structure for the active site.