A new class of Mo/Fe/S clusters with the MoFe3S3 core has been synthesized in attempts to model the FeMo-cofactor in nitrogenase. These clusters are obtained in reactions of the (Cl-4-cat)(2)Mo-2-Fe6S8(PR3)(6) [R = Et (I), Pr-n (II)] clusters with CO. The new clusters include those preliminarily reported: (Cl-4-cat)MoFe3S3(PEt3)(2)(CO)(5) (III), (Cl-4-cat)(O)MoFe3S3(PEt3)(3)(CO)(5) (IV), (Cl-4-cat)(Pyr)MoFe3S3(PEt3)(2)(CO)(6) (VI), and (Cl-4-cat)(Pyr)MoFe3S3(P(n)pr(3))(3)(CO)(4) (VIII). In addition the new (Cl-4-cat)(O)MoFe3S3((PPr3)-Pr-n)(3)(CO)(5) cluster (IVa), the (Cl-4-cat)(O)MoFe3S3(PEt3)(2)(CO)(6)cluster (V), the (Cl-4-cat)(O)MoFe3S3((PPr3)-Pr-n)(2)(CO)(6) cluster (Va), the (Cl-4-cat)(Pyr)MoFe3S3((PPr3)-Pr-n)(2)(CO)(6) cluster (VIa), and the (Cl-4-cat)((PPr3)-Pr-n)MoFe3S3((PPr3)-Pr-n)(2)(CO)(6) cluster (VII) also are reported, Clusters III-VIII have been structurally and spectroscopically characterized, EPR, zero-field Fe-57-Mossbauer spectroscopic characterizations, and magnetic susceptibility measurements have been used for a tentative assignment of the electronic and oxidation states of the MoFe3S3 sulfur-voided cuboidal clusters. A structural comparison of the clusters with the MoFe3S3 subunit of the FeMo-cofactor has led to the suggestion that the storage of reducing equivalents into M - M bonds, and their use in the reduction of substrates, may occur with the FeMo-cofactor, which also appears to have M-M bonding. On the basis of this argument, a possible N-2-binding and reduction mechanism on the FeMoco-cofactor is proposed.