From the reaction of cis-[(cyclam)Fe III (Cl) 2 ]Cl (cyclam ) 1,4,8,11-tetraazacyclotetradecane) with hydroxylamine in water the octahedral nitrosyliron complexes trans-[(cyclam)Fe(NO)Cl](ClO 4 ) (1) and cis-[(cyclam)Fe(NO)I]I (2) have been isolated as crystalline solids. EPR spectroscopy and variable-temperature susceptibility measurements established that 1 possesses an S ) 1 / 2 and 2 an S ) 3 / 2 ground state; both species are of the {Fe-NO} 7 type. Electrochemically, 1 can be reversibly one-electron oxidized yielding trans-[(cyclam)-Fe(NO)Cl] 2+ , an {Fe-NO} 6 species, and one-electron reduced yielding trans-[(cyclam)Fe(NO)Cl] 0 , an {Fe-NO} 8 species. These complexes have been characterized in CH 3 CN solutions by UV-vis and EPR spectroscopy; both possess a singlet ground state. All of these nitrosyliron complexes, including [LFe(NO)(N 3 ) 2 ] (S ) 3 / 2 ; L ) 1,4,7-trimethyl-1,4,7-triazacyclononane) and [L′Fe(NO)(ONO)(NO 2 )](ClO 4 ) (S ) 0; L′ ) 1,4,7triazacyclononane), have been studied by variable-temperature Mo ¨ssbauer spectroscopy both in zero and applied fields. The oxidation of 1 is best described as metal-centered yielding a complex with an Fe IV (S ) 1) coupled antiferromagnetically to an NO -(S ) 1), whereas its reduction is ligand-centered and yields a species with a low-spin ferric ion (S ) 1 / 2 ) antiferromagnetically coupled to an NO 2-(S ) 1 / 2 ). In agreement with Solomon et al. (J. Am. Chem. Soc. 1995, 117, 715) both {Fe-NO} 7 (S ) 3 / 2 ) species in this work are described as high-spin ferric (S ) 5 / 2 ) antiferromagnetically coupled to an NO -(S ) 1). Complex 1 is proposed to contain an intermediate spin ferric ion (S ) 3 / 2 ) antiferromagnetically coupled to NO -(S ) 1). The alternative descriptions as low-spin ferric antiferromagnetically coupled to NO -(S ) 1) or low-spin ferric with an NO -(S ) 0) ligand are ruled out by the applied field Mo ¨ssbauer spectra.

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. Mössbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.

The synthesis of a potentially redox active tripodal ligand containing a tris(aryloxide) functionalized mesitylene anchor, (((tBu)ArOH)(3)mes) (1), and its metalation with low-valent uranium to form [(((tBu)ArO)(3)mes)U] (1-U) is reported. The results from characterization by X-ray crystallography, spectroscopic studies, and computational analysis, as well as initial reactivity studies, support a +3 uranium oxidation state. Comparison to the previously synthesized complex, [(((tBu)ArO)(3)tacn)U] (2-U), featuring the redox-innocent triazacyclononane anchor reveals that changing the anchor from the flexible triazacyclononane to a rigid mesityl fragment increases the structural flexibility of the aryloxide substituents in complexes of 1. The synthesis and crystal structures of uranium(IV) amide complexes of 1-U and 2-U are discussed.

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