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A study is conducted to demonstrate that high exposed lead is vulnerable to metal ion displacement on reaction with zinc which results in a regular square pyramidal coordination about zinc within a [N (sub 2)S(sub 2)Zn](sub 2). Analysis of the two monomeric subunits accounted for the stability of the zinc structure over that of the lead.

An analysis of the transformation of sulfur site adducts of sulfur dioxide in nickel-bound thiolates to sulfate demonstrates the quick uptake of SO2 by (N, N'-bis(mercaptoethyl)-1,5-diazacyclooctane)-nickel(II), (bme-daco)Ni(II) or 1. SO2, 3.SO2 and 2.SO2 and 4.SO2 are good reversible SO2 binding agents. 1.SO2 acts as a store for SO2 in the solid state. Dry solids are secreted by 2.SO2 and 4.SO2.

The EPR-silent trinitrosyliron complex (TNIC or [{Fe[(NO).sub.3]}.sup.10]) has released gaseous NO, producing in the case of S[R.sup.-] = S[Ph.sup.-] the EPR-active, neutral dinitrosyliron complex (DNIC) (IMes)Fe(SPh)[(NO).sub.2]. The solid-state molecular structures of the N-heterocyclic carbene (NHC)-containing trinitrosyliron complexes have shown that all thee complexes have a tetrahedral geometry in which the bulky mesitylene substituents of the carbene ligand appear to umbrella the Fe[(NO).sub.2]L (L = NO, CO, SPh) motif.

Two cyclodextrin molecules which surround a 2Fe2S synthetic analogue of the active site, outfitted with an aryl sulfonate to promote inclusion into [beta]-cyclodextrin is used to imitate the hydrophobic cavity of the active site of [FeFe]-Hydrogenase. The X-ray crystal structure of the clathrate which showed an increased torsion angle between the apical CO ligands indicated that the supramolecular cage destabilizes the eclipsed geometry typical of diiron model complexes.

The zinc derivatives and an NiZn heterometallic compound using (BME-DACO)NiII as a metalothiolate ligand were prepared. The H2BME-DACO ligand in toluene solution was reacted with anhydrous zinc acetylacetonate to give ((BME-DACO)Zn)2. Meanwhile, (((BME-DACO)Ni)3Zn2Cl2)(BF4)2 was generated from the reaction of (BME-DACO)NiII and NaBF4 in methanol with a THF solution of ZnCl2. The resulting compounds are air-stable, with the former compound discoloring with prolonged exposure.

A study was conducted to explore the site specificity (sulfur vs the Fe-Fe bond) of oxygenation of diiron ([Fe.sup.I][Fe.sup.I] and [Fe.sup.II][Fe.sup.II]) organometallics that model the 2-iron subsite in the [FeFe]-hydrogenase active site. The proton/electron-coupled process of deoxygenation and the possible biological relevance of oxygenation and deoxygenation studies is also discussed.

N-heterocyclic carbene ligands with different steric bulk on the N-substituents are used to synthesize and characterize a series of asymmetrically disubstituted models of the active site of [FeFe]-hydrogenase. The results for [FeFe]-hydrogenase active site model provides insight into the features that contribute to the stability of the unique rotated structure of the [Fe.sup.I][Fe.sup.II] mixed valent [H.sub.ox] mimics.

The selective synthesis of two isomeric diiron dithiolate complexes, an unsymmetrically chelating complex ([mu]-pdt)[Fe[(CO).sub.3]][Fe(CO)([K.sup.2]-PNP)] and a symmterically bridging complex ([mu]-pdt)([mu]-PNP)][[Fe[(CO).sub.2]].sub.2] (PNP = [(EtO).sub.2]PN(Me)P[(OEt).sub.2] is described. The results from reaction kinetics provide evidence for the lability of the CO ligands in the Fe[(CO).sub.3] unit of the unsymmetrically substituted diiron dithiolate complex.

Two representative platinum compounds [[Pt(dien)Cl].sup.+] and [[Pt(terpy)Cl].sup.+] (where dien = diethylenetriamine, terpy = 2,2':6,2'-terpyridine) with [[Zn(bme-dach)].sub.2] are studied. The direct synthesis of the Zn-Pt thiolate-bridged species and the Pt([N.sub.2][S.sub.2]) chelate, where Pt has displaced the Zn from the chelate core, has permitted the isolation of X-ray quality crystals to confirm the bridging and metal-exchanged structures.

Infrared (IR) spectroscopy is used for studying the iron-iron hydrogenase ([FeFe][H.sub.2]ase) active site. The comparisons of the computed IR spectra of the [super 12]CO and [super 13]CO inhibited form with the experimental IR spectra has shown that exogenous CO binds terminally to the distal iron center.

Heterobimetallic complexes comprised of W[(CO).sub.4]) adducts of ([N.sub.2][S.sub.2])M(NO) are isolated and characterized by v(CO) and v(NO) infrared (IR) spectroscopies and X-ray diffraction. Cyclic voltammetry studies have shown similar reduction potentials of the (bme-dach)Co(NO) and (bme-dach)Fe(NO) free metalloligands, which are positively shifted by ca. 0.61 and 0.48 V, respectively, upon complexation to W[(CO).sub.4].

The isolation and structural characterization of a sulfinato species, [[Et.sub.4]N].sub.2][Ni(ema).[O.sub.2]], prepared through a unique direct reaction of molecular [O.sub.2] with crystalline [[Et.sub.4]N].sub.2][Ni(ema)] is described.

A stable [Fe.sup.II][Fe.sup.I] paramagnetic [H.sub.2]ase model complex with semi-bridging CO is synthesized that has modeled the 2Fe2S, mixed valent, subsite of the H-cluster in [FeFe][H.sub.2]ase. It is shown that the unpaired spin density and the singly occupied molecular orbital (SOMO) are on the rotated iron and the open site in the mixed-valent, resting form of the enzyme is readily activated by redox changes to perform both hydrogenase functions, [H.sup.+] reduction and [H.sub.2] binding and oxidation.

Density functional theory computations are used for the rational design of synthetic di-iron complexes as structural models of the reduced form of the iron-iron hydrogenase enzyme active site. The use of five-atom S-to-S linkers or the attachment of pendant functionalities to a three-carbon framework and/or asymmetric substitution of donor ligands is s synthetic strategy that is predicted to stabilize an orientation of the diatomic ligands that is more similar to that observed in the enzyme.

An N-heterocyclic carbene containing [FeFe]H2ase model complex shows an unexpected two-electron reduction to be involved in its electro-catalytic dihydrogen production. The density functional calculations show that the aryl-substituted N-heterocyclic carbene can accept second electron more readily than the Fe-Fe manifold and the juxtaposition of these two one-electron reductions resembles the [FeFe]H2ase active site with a FeFe di-iron unit joined to be the electroactive 4Fe4S cluster.

The quantification of SO2 binding abilities in the Ni-1 and Ni-1' complexes and the isolation and characterization of the Ni-1'.2SO2 and Ni-1'.SO2 adducts is reported. The uptake of sulphur dioxide by the dithiolato nickel complex, Ni-1' results in a crystalline material in which thiolate-sulfur to SO2-sulfur interactions are well defined by X-ray crystallography, nu-(SO) IR spectroscopy and thermal gravimetric analysis.

The current experiment favors a mechanism in which methylation occurs first to NiP0- or NiP!-[FeSo4+, followed by coordination of CO to form NiPII(CO)(CH3) which breaks one of the SNi4 bonds. Density functional calculations are used to address the controversies related to the experiments

Research has been conducted on hydrogenase enzymes. The authors report that the hydrogen uptake in the enzymes can be investigated via H/D exchange reactivity, and that diiron(I) complexes serve as structural models for the iron hydrogenase active site.

The thiolate ligands of [NiFe]-H2ase enzymes have been implicated as proton-binding sites for the reduction/oxidation of H+/H2. This study examines the ligand effect on reactivity of NiN2S2 complexes with an array of acids in methanol solution. UV–Vis absorption spectroscopy is utilized to observe the transformation from the monomeric species to a trimetallic complex that is formed after proton-induced ligand dissociation. Nickel complexes with a flexible (propyl and ethyl) N to N linker were found to readily form the trimetallic complex with acids as weak as ammonium (pKa = 10.9 in methanol). A more constrained nickel complex with a diazacycloheptane N to N linker required stronger acids such as 2,2-dichloroacetic acid (pKa = 6.38 in methanol) to form the trimetallic complex and featured the formation of an NiN2S2H+ complex with acetic acid (pKa = 9.63 in methanol). The most strained ligand, which featured a diazacyclohexane backbone, readily dissociated from the nickel center upon mixture with acids with pKa ≤ 9.63 and showed no evidence of a trimetallic species with any acid. This research highlights the dramatic differences in reactivity with proton sources that can be imparted by minor alterations to ligand geometry and strain. [ABSTRACT FROM AUTHOR]

Synthetic analogues of the catalytic subsite of the hydrogen-producing enzyme HydA1 have been disappointingly inactive. The incorporation of such analogues into the enzyme's active site reveals the requirements for activity. [...]

The nitrosylated diiron complexes, Fe2(NO)3, of this study are interpreted as a mono‐nitrosyl Fe(NO) unit, MNIU, within an N2S2 ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N2S2⋅Fe(NO)2+ complex, 1+, of Enemark–Feltham electronic notation {Fe(NO)}7‐{Fe(NO)2}9, is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}7‐{Fe(NO)2}10, 10, and {Fe(NO)}8‐{Fe(NO)2}10, 1−, were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S=1/2 , complex, 10, finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe‐N‐O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe‐N‐O angle to 171° in 1−. Compensating shifts in FeMNIU distances from the N2S2 plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1− is a triplet‐state {Fe(NO)}8 with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment. The remarkable stability of the [Fe(NO)N2S2‐Fe(NO)2] species that permits isolation and structural characterization in its three redox levels relates to the delocalization of charge on both Fe(NO) and the Fe(NO)2 support. Subtle shifts of Fe(NO) from the N2S2 base accommodates a rare, non‐heme {Fe(NO)}8 triplet state and ca. linear. [ABSTRACT FROM AUTHOR]