Nitrogenase consists of two proteins: an iron containing protein and an iron and molybdenum (FeMo) containing protedl,21. Recent analyses[3] of MO EXAFS data of the FeMo protein and the FeMo

Nitrogenase consists of two proteins: an iron containing protein and an iron and molybdenum (FeMo) containing protedl,21. Recent analyses[3] of MO EXAFS data of the FeMo protein and the FeMo COfactod41 isolated from it have led to the conclusion that the Mo atom is prinprily coordinated to sulfur and is separated by < 3 A from another metal atom, not Mo and,therefore, Fe, and that the Mo coordination in native proteins is similar o r identical to that in the FeMo cofactor. Together with the known presence of inorganic sulfide and cysteinyl residues in FeMo proteins, it is probable that the active site consists of a Mo-Fe-S(R) polynuclear cluster with metal bridging by sulfide. Using a reaction system consisting of FeC13/(R4N)2 M S4/RtSH in alcohol solvents, a series of "double cubane" complexes(1) [ M ~ F ~ ~ s ~ ( s E ~ ) ~ ] ~ ' , (11) [ M O ~ F ~ ~ S ~ ( S E ~ ) ~ ] ~ -, (111) [ M ~ F ~ ~ s ~ ( s E ~ ) ~ ~ ] ~ and (IV) [ M F ~ ~ S ~ ( S C H ~ W I ~ ~ ' have been isolated [5,6,71, M = Mo, W. The structures consist of bridged arrangements of individual M O F ~ ~ S ~ ( S R ' ) ~ clusters. In 111 and in IV, the bridge i s M(p~-sEt)3Fe(p~-SEt)~M (Fig. 1) and M (I.12-SCH2Ph)3Fe(h-SCH2Ph)3 M, respectively. In I and 11, the bridge is M (SEt)ZS M and Mo(SE~)~MO, respectively. The zero field M8ssbauer spectrum of (I) a t 77 and 4.2 K consists of a single slightly asymmetric quadrupole doublet with broadened lines, with the magnitude of the quadrupole splitting and the line width increasing from 77 to 4.2 K. Theoretical fits were made assurnlng one quadrupole pair with Lorentzian line shapes in each spectrum and the quadrupole splitting and isomer shifts are listed in Table I. The zero field spectrum of I1 at 77 and 4.2 K consists of two partially resolved quadrupole doublets, with quadrupole splittings and line -widths increasing from 77 to 4.2 K for both doublets. The theoretical fits with unconstrained linewidths, intensities and positions indicated approximately 1 :1 relative integrated intensities for the two doublets at both temperatures. At 77 K, the doublet with larger splitting had slightIy greater linewidths. MClssbauer spectra of 111 and IV are similar to I and II, but have an additional component due to the Fe bridging atom (Fig. 2). In both cases, the additional component has an integrated intensity which is approximately 1/6 that of the spectrum due to the cubane clusters, as expected from the structure. In 111, the parameters a re consistent with an assignment of low spin ferric for the bridging iron, while in IV, the parameters are consistent with high spin ferrous iron. The spectral shapes indicate inequivalent iron sites in the clusters at low temperature, and some sensitivity of the cluster geometry to the nature of Rand R'. However, isomer shifts and quadrupole splittings for iron atoms within the clusters a re very similar,and are similar from cluster to cluster. The isomer shifts a re observed to fall in the range 0.27 0.31 mm/sec (relative to iron metal at the same temperature) and a re nearly independent of M=Mo or W. Interpolation of the isomer shift with the nearly linear dependence of isomer shifts of tetrahedral FeSq units on Fe '