Reactivity of M(II) metal-substituted derivatives of pig purple acid phosphatase (uteroferrin) with phosphate.

The Fe(II) of the binuclear Fe(II)Fe(III) active site of pig purple acid phosphatase (uteroferrin) has been replaced in turn by five M(II) ions (Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)). An uptake of 1 equiv of M(II) is observed in all cases except that of Cu(II), when a second more loosely bound Cu(II) is removed by treatment with edta. The products have been characterized by different analytical procedures and by UV-vis spectrophotometry. At 25 degrees C, I = 0.100 M (NaCl), the nonenzymatic reactions with H(2)PO(4)(-) give the mu-phosphato product, and formation constants K/M(-1) show an 8-fold spread at pH 4.9 of 740 (Mn), 165 (Fe), 190 (Co), 90 (Ni), 800 (Cu), 380 (Zn). The variations in K correlate well with stability constants for the complexing of H(2)PO(4)(-) and (CH(3)O)HPO(3)(-) with M(II) hexaaqua ions. At pH 4.9 with [H(2)PO(4)(-)] > or = 3.5 mM rate constants k(obs) decrease, and an inhibition process in which a second [H(2)PO(4)(-)] coordinates to the dinuclear center is proposed. The mechanism considered accounts for most but not all of the features displayed. Thus K(1) values for the coordination of phosphate to M(II) are in the range10-60 M(-1), whereas K(2) values for the bridging of the phosphate to Fe(III) are in the narrower range 7.8-12.4. From the fits described K(i) approximately 10(3) M(-1) for the inhibition step, which is independent of the identity of M(II). Values of k(obs) decrease with increasing pH, giving pK(a) values which are close to 3.8 and independent of M(II) (Fe(II), Zn(II), Mn(II)). The acid dissociation process is assigned to Fe(III)-OH(2) to Fe(III)-OH(-), where OH(-) is less readily displaced by phosphate.

Pulse radiolysis studies on galactose oxidase.

Single-Cu-containing galactose oxidase in the GOase(semi) state (Cu(II), no Tyr(*) radical) reacts with pulse radiolysis generated formate radicals CO(2)(*-) to give an intermediate UV-vis spectrum assigned as RSSR(*-), peak at 450 nm (epsilon = 8100 M(-1) cm(-1)). From a detailed kinetic analysis at 450 nm, pH 7.0, the following steps have been identified. First the strongly reducing CO(2)(*-) (-1.9V) reduces GOase(semi) (k(0) > or = 6.5 x 10(8) M(-1) s(-1)) to a species GOase(semi)(*-). This is followed by biphasic reactions (i) GOase(semi)(*-) + GOase(semi) (k(1) = 1.6 x 10(7) M(-1) s(-1)) to give GOase(semi) + P(*-) and (ii) P(*-) + GOase(semi) (k(2) = 6.7 x 10(6) M(-1) s(-1)) to give GOase(semi)RSSR(*-). There are no significant absorbance changes for the formation of GOase(semi)(*-) and P(*-), which are Cu(I) (or related) species. However, GOase(semi)RSSR(*-) has an absorption spectrum which differs significantly from that of GOase(semi). The 450 nm peak is characteristic of an RSSR(*-) radical with two cysteines in close sequence proximity and is here assigned to Cys515-Cys518, which is at the GOase surface and 10.2 A from the Cu. On chemical modification of the RSSR group with HSPO(3)(2-) to give RSSPO(3)H(-) and RS(-), absorbance changes are approximately 50% of those previously observed. The decay of RSSR(*-) (0.17 s(-1)) results in the formation of GOase(red). No RSSR(*-) formation is observed in the reaction of GOase(semi) Tyr495Phe with CO(2)(*-), and a single process giving GOase(red)Tyr495Phe occurs. Similarly in the reaction of GOase(ox) with CO(2)(*-), a single-stage reaction gives GOase(semi).

Preparation and properties of the aqua ions [W(4)S(4)(H(2)O)(12)](n+) (n = 5, 6) and crystal structure of (Me(2)NH(2))(6)[W(4)S(4)(NCS)(12)].0.5H(2)O.

The [3 + 1] reaction of [W(3)S(4)(H(2)O)(9)](4+) with [W(CO)(6)] in 2 M HCl under hydrothermal conditions (130 degrees C) gives the [W(4)S(4)(H(2)O)(12)](6+) cuboidal cluster, reduction potential 35 mV vs NHE (6+/5+ couple). The reduced form is obtained by controlled potential electrolysis. X-ray crystal structure was determined for (Me(2)NH(2))(6)[W(4)S(4)(NCS)(12)].0.5H(2)O. The W-W and W-S bond lengths are 2.840 and 2.379 A, respectively.

Synthesis, Structures, and Redox Properties of Octa(&mgr;(3)-sulfido)hexarhenium(III) Complexes Having Terminal Pyridine Ligands.

A series of &mgr;(3)-sulfido Re-Re bonded octahedral hexarhenium(III) clusters having mixed chloride-pyridine (py) or -4-cyanopyridine (cpy) terminal ligands, [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(py)(2)}] (Bu(4)N(+) = tetra-n-butylammonium cation) (1a), [Bu(4)N](2)[cis-{Re(6)S(8)Cl(4)(py)(2)}] (1b), [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a), [Bu(4)N](2)[cis-{Re(6)S(8)Cl(4)(cpy)(2)}] (2b), and [Bu(4)N][mer-{Re(6)S(8)Cl(3)(py)(3)}] (3), and their one-electron-oxidized Re(III)(5)Re(IV) species, [Bu(4)N][trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a'), [Bu(4)N][trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a'), and mer-[Re(6)S(8)Cl(3)(py)(3)] (3'), have been prepared and characterized by several physical methods. X-ray crystallographic studies for 1a, 2a, and 3 showed that the Re(6)S(8) core structures are not significantly affected by the type and number of pyridyl ligands. The mixed valent cluster 1a' is of a structurally delocalized type, structural parameters being very similar to those of 1a. Cyclic voltammograms in acetonitrile showed that there is no distinct difference in the redox potentials (Re(III)(6)/Re(III)(5)Re(IV)) between the cis and the corresponding trans isomers. Both 1a and 1b show a reversible redox wave at 0.77 V vs Ag/AgCl. Redox potentials are more positive for 2a and 2b (0.83 V) and 3 (0.97 V). Clusters 2a and 2b show two-step ligand-centered redox waves at -1.19 and -1.28 V, and -1.18 and -1.29 V, respectively. Temperature-dependent magnetic susceptibility measurements have revealed that 1a' and 3' have an S = 1/2 ground state. The electron self-exchange rate constant for the reaction of 2a with 2a' in dichloromethane as obtained by (1)H NMR line-broadening method is 1.2 x 10(9) M(-)(1) s(-)(1) (298.2 K) with DeltaH() = 30.2 +/- 2.1 kJ mol(-)(1) and DeltaS() = 30 +/- 8 J mol(-)(1) K(-)(1). It has been suggested that the previously reported protonated species [Re(6)S(7)(SH)Cl(6)](3)(-) would actually be a one-electron-oxidized [Re(6)S(8)Cl(6)](3)(-). Crystal data: [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a), monoclinic, space group C2/c, a = 24.693(8) Å, b = 19.494(4) Å, c = 18.592(4) Å, beta = 115.76(2) degrees, Z = 4; [Bu(4)N](2)[trans-{Re(6)S(8)Cl(4)(cpy)(2)}] (2a), orthorhombic, space group Cmca, a = 19.304(3) Å, b = 17.894(7) Å, c = 18.773(4) Å, Z = 4; [Bu(4)N][mer-{Re(6)S(8)Cl(3)(py)(3)}] (3), monoclinic, space group P2(1)/n, a = 16.156(5) Å, b = 19.760(5) Å, c = 18.895(4) Å, beta = 108.94(2) degrees, Z = 4; [Bu(4)N][trans-{Re(6)S(8)Cl(4)(py)(2)}] (1a'), monoclinic, space group C2/c, a = 20.524(5) Å, b = 13.794(4) Å, c = 16.399(4) Å, beta = 109.72(2) degrees, Z = 4.

Preparation and Properties of Group 13 (Ga, In, Tl) Heterometallic Single and Corner-Shared Double Cube Derivatives of [Mo(3)S(4)(H(2)O)(9)](4+) and Related Studies.

New routes are described for the preparation of cuboidal complexes [Mo(3)InS(4)(H(2)O)(12)](5+), [Mo(3)GaS(4)(H(2)O)(12)](5+), and [Mo(6)InS(8)(H(2)O)(18)](8+) from the incomplete cuboidal [Mo(3)S(4)(H(2)O)(9)](4+). A comparison of the aqueous solution properties of the single cubes, [Mo(3)GaS(4)(H(2)O)(12)](5+) and [Mo(3)InS(4)(H(2)O)(12)](5+), and the double cubes, [Mo(6)InS(8)(H(2)O)(18)](8+) and [Mo(6)TlS(8)(H(2)O)(18)](8+), the total listing of group 13 derivatives, is reported. The single cube [Mo(3)InS(4)(H(2)O)(12)](5+) can be quantitatively converted into the double cube by reductive (H(3)PO(2) or BH(4)(-)) addition of [Mo(3)S(4)(H(2)O)(9)](4+). The double cubes are O(2) sensitive, much more so in HCl than Hpts (pts(-) = p-toluenesulfonate), and are oxidized by H(+) in HCl solutions with the formation of H(2) (detected by gas chromatography). The single cubes are less reactive in air, and are only oxidized by H(+) at higher ( approximately 4 M) levels. Stoichiometry measurements with [Co(dipic)(2)](-) and [Fe(H(2)O)(6)](3+) as 1e(-) oxidants were used to confirm charges on the clusters. In the absence of reduction potentials, rate constants for outer-sphere [Co(dipic)(2)](-) oxidations give a measure of redox properties. Oxidation of [Mo(6)InS(8)(H(2)O)(18)](8+) with [Co(dipic)(2)](-) provides a route back to the single cube [Mo(3)InS(4)(H(2)O)(12)](5+). Properties of [W(3)InS(4)(H(2)O)(12)](5+) and [Mo(6)InO(2)S(6)(H(2)O)(18)](8+) obtained from [W(3)S(4)(H(2)O)(9)](4+) and [Mo(3)OS(3)-(H(2)O)(9)](4+) are also considered. The single cube [W(3)InS(4)(H(2)O)(12)](5+) is >10(3) times more reactive with [Co(dipic)(2)](-) than [Mo(3)InS(4)(H(2)O)(12)](5+), consistent with reducing properties W > Mo. No evidence was obtained in these studies for [Mo(3)TlS(4) (H(2)O)(12)](5+), [Mo(6)GaS(8)(H(2)O)(18)](8+), or the recently proposed 6+ analogue of [Mo(3)GaS(4)(H(2)O)(12)](5+).

Preparation and Solution Properties of Chalcogenide-Rich Clusters [Mo(3)Y(7)(H(2)O)(6)](4+) (Y = S, Se): Kinetics of PR(3)(3-) Abstraction of Y from &mgr;-(Y(2)(2-)) and H(2)O Substitution by Cl(-) and Br(-).

The chalcogenide-rich trinuclear Mo(IV)(3) clusters [Mo(3)Y(7)(H(2)O)(6)](4+), containing single &mgr;(3)-(Y(2)(-)) and three &mgr;-(Y(2)(2)(-)) core ligands, have been obtained for the first time from polymeric {Mo(3)Y(7)Br(4)}(x)() via [Mo(3)Y(7)Br(6)](2)(-) (Y = S, Se). ICP analyses of 2 M HCl solutions give Mo:S and Mo:Se ratios consistent with the formulas indicated, and on reaction with concentrated HBr, 85% recovery of (Et(4)N)(2)[Mo(3)S(7)Br(6)], the structure of which is known, has been achieved. Abstraction of S and Se with PPh(3) (two-phase system), or the water-soluble phosphine (3-SO(3)C(6)H(4))(3)P(3)(-) (PR(3)(3)(-)), gives quantitative formation of [Mo(3)S(4)(H(2)O)(9)](4+) and [Mo(3)Se(4)(H(2)O)(9)](4+). With CN(-), both abstraction of S (or Se) and CN(-) replacement of H(2)O is observed, giving [Mo(3)S(4)(CN)(9)](5)(-) and [Mo(3)Se(4)(CN)(9)](5)(-) as products. It was possible to assign which atom of the sideways eta(2),eta(2) &mgr;-(S(2)(2)(-)) and &mgr;-(Se(2)(2)(-)) ligands is abstracted using the structurally characterized [Mo(3)S(4)Se(3)(H(2)O)(6)](4+) cluster. Thus it was demonstrated that with the phosphines the equatorial (to the Mo(3) plane) Se atoms of the three &mgr;-(SSe(2)(-)) groups are removed with formation of the Mo(3)S(4)(4+) core. Kinetic studies on the reactions of [Mo(3)S(7)(H(2)O)(6)](4+) and [Mo(3)Se(7)(H(2)O)(6)](4+) with PR(3)(3)(-) give approximately 10(3) faster abstraction rate constants (k(a)/M(-)(1) s(-)(1)) for S than Se. The rate law k(a) = k(1)[H(+)] + k(-)(1)[H(+)](-)(1) is explained by the involvement of protonated &mgr;-(Y(2)(2)(-)) (k(1)) and an H(2)O conjugate-base form (k(-)(1)). Equilibration rate constants for X(-) = Cl(-) and Br(-) substitution of H(2)O on [Mo(3)S(7)(H(2)O)(6)](4+) and [Mo(3)Se(7)(H(2)O)(6)](4+) are however independent of [H(+)] in the range 0.5-2.0 M investigated. With X(-) concentrations up to 1.3 M (S cluster) and 0.3 M (Se), the uniphasic reactions are assigned as substitution of the H(2)O cis to &mgr;(3)-(Y(2)(-)) at each Mo. At 25 degrees C formation rate constants 10(4)k(f)/M(-)(1) s(-)(1) are as follows for [Mo(3)S(7)(H(2)O)(6)](4+): Cl(-) (1.83); Br(-) (2.07). The same rate constants are as follows for [Mo(3)Se(7)(H(2)O)(6)](4+): Cl(-) (6.7); Br(-) (33). Formation rate constants for Cl(-) are surprisingly 2 x 10(6) times slower than for the reaction of [Mo(3)S(4)(H(2)O)(9)](4+) with Cl(-). Reactions of Mo(3)S(7)(4+) with three metals (Sn, Ni, In) were studied briefly.

Formation, Properties, and Characterization of a Fully Reduced Fe(II)Fe(II) Form of Spinach (and Parsley) [2Fe-2S] Ferredoxin with the Macrocyclic Complex [Cr(15-aneN(4))(H(2)O)(2)](2+) as Reductant.

Reduction of spinach and parsley ferredoxin FdI in the Fe(III)Fe(III) state with the 1,4,8,12-tetraazacyclopentadecane complex [Cr(15-aneN(4))(H(2)O)(2)](2+), here written as Cr(II)L, provides the first evidence for two 1-equiv steps yielding an Fe(II)Fe(II) product. Rate constants (25 degrees C) for spinach FdI are 2760 and 660 M(-)(1) s(-)(1), respectively, at pH 7.5, I = 0.100 M (NaCl). An important observation is that the Cr(III)L generated in the first step remains attached to the Fe(II)Fe(III) product and perturbs the protein active site sufficiently to make the second stage possible. The second Cr(II)L reduction is of the "outer-sphere" type, and the Cr(III)L generated is not attached to the protein. Anaerobic reoxidation of the fully reduced protein with [Co(NH(3))(6)](3+) is rapid and can be achieved with approximately 80% recovery of the Fe(III)Fe(III) oxidation state over 40 min. Air oxidation yields the Cr(III)L product Fe(III)Fe(III).Cr(III)L (Fe:Cr = 2:1). With Anabaena variabilis only a one-step reduction is observed and there is no Cr(III)L attachment. From a comparison of amino acid sequences with spinach (and parsley) FdI, a likely point of Cr(III)L attachment is indicated. Comparisons are made with dithionite as reductant. In addition, square-wave voltammetry on spinach Fe(III)Fe(III).Cr(III)L gives two reduction potentials -273 and -410 mV vs NHE. The different redox products have been characterized by EPR. Using (1)H NMR line-broadening techniques, evidence for Cr(III)L binding at a surface site close to Tyr-25/Tyr-82 is obtained. Also from investigations with redox-inactive [Cr(en)(3)](3+) as a competitive inhibitor for Cr(II)L reduction of spinach Fe(III)Fe(III), Tyr-25/Tyr-82 is proposed as the site for Cr(II)L reduction. From an extension of studies to include reduction of Fe(III)Fe(III).Cr(III)L with Cr(II)L, evidence is obtained for a second reaction site when that at Tyr-25/Tyr-82 is no longer available.

Kinetic Studies on the Redox Interconversion of GOase(semi) and GOase(ox) Forms of Galactose Oxidase with Inorganic Complexes as Redox Partners.

Redox interconversions between the GOase(semi) (Cu(II), Tyr) and tyrosyl radical containing GOase(ox) (Cu(II), Tyr(*)) oxidation states of the Cu-containing enzyme galactose oxidase (GOase) from Fusarium NRRL 2903 have been studied. The inorganic complexes [Fe(CN)(6)](3)(-) (410 mV), [Co(phen)(3)](3+) (370 mV), [W(CN)(8)](3)(-) (530 mV), and [Co(dipic)(2)](-) (362 mV) (E degrees ' values vs NHE; dipic = 2,6-dicarboxylatopyridine) were used as oxidants for GOase(semi), and [Fe(CN)(6)](4)(-) and [Co(phen)(3)](2+) as reductants for GOase(ox). On oxidation of GOase(semi) a radical is generated at the coordinated phenolate of Tyr-272 to give GOase(ox). The one-electron reduction potential E degrees ' (25 degrees C) for the GOase(ox)/GOase(semi) couple varies with pH and is 400 mV vs NHE at pH 7.5, the smallest value so far observed for a tyrosyl radical. The reactions are very sensitive to pH, or more precisely to pK(a) values of GOase(semi) and GOase(ox), and the charge on the inorganic reagent. For example, with [Fe(CN)(6)](3)(-) as oxidant, the rate constant (25 degrees C)/M(-)(1) s(-)(1) of 0.16 x 10(3) (pH approximately 9.5) increases to 4.3 x 10(3) (pH approximately 5.5), while for [Co(phen)(3)](3+) a value of 4.9 x 10(3) (pH approximately 9.5) decreases to 0.04 x 10(3) (pH approximately 5.5), I = 0.100 M (NaCl). From the kinetics a single GOase(semi) acid dissociation process, pK(a) = 8.0 (average), has been confirmed by UV-vis spectrophotometric studies (7.9). The corresponding value for GOase(ox) is 6.7. No comparable kinetic or spectrophotometric pH dependences are observed with the Tyr495Phe variant, indicating the axial Tyr-495 as the site of protonation. Neutral CH(3)CO(2)H and HN(3) species bind at the substrate binding site of GOase(semi), thus mimicking the behavior of primary alcohols RCH(2)OH, the natural substrate of GOase. On coordination, loss of a proton occurs, and inhibition of the oxidation with [Fe(CN)(6)](3)(-) is observed.

Interconversion and Reactivity of Two Heterometallic Tin-Containing Cuboidal Clusters from [Mo(3)S(4)(H(2)O)(9)](4+): X-ray Structure of the Single Cube with an Mo(3)SnS(4) Core.

The Mo(3)SnS(4)(6+) single cube is obtained by direct addition of Sn(2+) to [Mo(3)S(4)(H(2)O)(9)](4+). UV-vis spectra of the product (0.13 mM) in 2.00 M HClO(4), Hpts, and HCl indicate a marked affinity of the Sn for Cl(-), with formation of the more strongly yellow [Mo(3)(SnCl(3))S(4)(H(2)O)(9)](3+) complex complete in as little as 0.050 M Cl(-). The X-ray crystal structure of (Me(2)NH(2))(6)[Mo(3)(SnCl(3))S(4)(NCS)(9)].0.5H(2)O has been determined and gives Mo-Mo (mean 2.730 Å) and Mo-Sn (mean 3.732 Å) distances, with a difference close to 1 Å. The red-purple double cube cation [Mo(6)SnS(8)(H(2)O)(18)](8+) is obtained by reacting Sn metal with [Mo(3)S(4)(H(2)O)(9)](4+). The double cube is also obtained in approximately 50% yield by BH(4)(-) reduction of a 1:1 mixture of [Mo(3)SnS(4)(H(2)O)(10)](6+) and [Mo(3)S(4)(H(2)O)(9)](4+). Conversely two-electron oxidation of [Mo(6)SnS(8)(H(2)O)(18)](8+) with [Co(dipic)(2)](-) or [Fe(H(2)O(6)](3+) gives the single cube [Mo(3)SnS(4)(H(2)O)(12)](6+) and [Mo(3)S(4)(H(2)O)(9)](4+) (up to 70% yield), followed by further two-electron oxidation to [Mo(3)S(4)(H(2)O)(9)](4+) and Sn(IV). The kinetics of the first stages have been studied using the stopped-flow method and give rate laws first order in [Mo(6)SnS(8)(H(2)O)(18)](8+) and the Co(III) or Fe(III) oxidant. The oxidation with [Co(dipic)(2)](-) has no [H(+)] dependence, [H(+)] = 0.50-2.00 M. With Fe(III) as oxidant, reaction steps involving [Fe(H(2)O)(6)](3+) and [Fe(H(2)O)(5)OH](2+) are implicated. At 25 degrees C and I = 2.00 M (Li(pts)) k(Co) is 14.9 M(-)(1) s(-)(1) and k(a) for the reaction of [Fe(H(2)O)(6)](3+) is 0.68 M(-)(1) s(-)(1) (both outer-sphere reactions). Reaction of Cu(2+) with the double but not the single cube is observed, yielding [Mo(3)CuS(4)(H(2)O)(10)](5+). A redox-controlled mechanism involving intermediate formation of Cu(+) and [Mo(3)S(4)(H(2)O)(9)](4+) accounts for the changes observed.

Aqueous Solution Chemistry of the Mo(3)PdS(4) Cube: Substitution Reactions and the Double to Single Cube Interconversion Induced by CO, Two Phosphines, Cl(-), Br(-), and NCS(-).

The kinetics of conversion of an edge-linked double cube, in this case [{Mo(3)PdS(4)(H(2)O)(9)}(2)](8+), to the corresponding single cube [Mo(3)(PdX)S(4)(H(2)O)(9)](4+), has been studied for the first time. Reaction is induced by six reagents X = CO, two water-soluble phosphines, Cl(-), Br(-), and NCS(-), which complex at the tetrahedral Pd. The first stage of reaction is fast and is accompanied by color changes, e.g. purple to dark blue in the case of Cl(-), assigned as double to single cube conversion. With X = CO and the two phosphines, when absorbance changes are intense enough for stopped-flow monitoring with reactants at Pd-SCN. On removal of e.g. Cl(-) by chromatography or addition of Ag(+), the double cube re-forms.

Preparation and Properties of the Corner-Shared Double Cube [Mo(6)BiS(8)(H(2)O)(18)](8+) as a Derivative of [Mo(3)S(4)(H(2)O)(9)](4+) in Aqueous Acidic Solutions.

The reaction of [Mo(3)S(4)(H(2)O)(9)](4+) with Bi(III) in the presence of BH(4)(-) (rapid), or with Bi metal shot (3-4 days), gives a heterometallic cluster product. The latter has been characterized as the corner-shared double cube [Mo(6)BiS(8)(H(2)O)(18)](8+) by the following procedures. Analyses by ICP-AES confirm the Mo:Bi:S ratio as 6:1:8. Elution from a cation-exchange column by 4 M Hpts (Hpts = p-toluenesulfonic acid), but not 2 M Hpts (or 4 M HClO(4)), is consistent with a high charge. The latter is confirmed as 8+ from the 3:1 stoichiometries observed for the oxidations with [Co(dipic)(2)](-) or [Fe(H(2)O)(6)](3+) yielding [Mo(3)S(4)(H(2)O)(9)](4+) and Bi(III) as products. Heterometallic clusters [Mo(6)MS(8)(H(2)O)(18)](8+) are now known for M = Hg, In, Tl, Sn, Pb, Sb, and Bi and are a feature of the P-block main group metals. The color of [Mo(6)BiS(8)(H(2)O)(18)](8+) in 2.0 M Hpts (turquoise) is different from that in 2.0 M HCl (green-blue). Kinetic studies (25 degrees C) for uptake of a single chloride k(f) = 0.80 M(-)(1) s(-)(1), I = 2.0 M (Hpts), and the high affinity for Cl(-) (K > 40 M(-)(1)) exceeds that observed for complexing at Mo. A specific heterometal interaction of the Cl(-) not observed in the case of other double cubes is indicated. The Cl(-) can be removed by cation-exchange chromatography with retention of the double-cube structure. Kinetic studies with [Co(dipic)(2)](-) and hexaaqua-Fe(III) as oxidants form part of a survey of redox properties of this and other clusters. The Cl(-) adduct is more readily oxidized by [Co(dipic)(2)](-) (factor of approximately 10) and is also more air sensitive.

Ligand Substitution Reactions at the Nickel of [Mo(3)NiS(4)(H(2)O)(10)](4+) with Two Water Soluble Phosphines, CO, Br(-), I(-), and NCS(-) and the Inertness of the 1,4,7-Triazacyclononane (L) Complex [Mo(3)(NiL)S(4)(H(2)O)(9)](4+).

Comparisons (25 degrees C) are made of substitution reactions, X replacing H(2)O, at the tetrahedral Ni of the heterometallic sulfido cuboidal cluster [Mo(3)NiS(4)(H(2)O)(10)](4+), I = 2.00 M (LiClO(4)). Stopped-flow formation rate constants (k(f)/M(-)(1) s(-)(1)) for six X reagents, including two water soluble air-stable phosphines, 1,3,5-triaza-7-phosphaadamantane PTA (119) and tris(3-sulfonatophenyl)phosphine TPPTS(3)(-) (58), and CO (0.66), Br(-) (14.6), I(-) (32.3), and NCS(-) (44) are reported alongside the previous value for Cl(-) (9.4). A dependence on [H(+)] is observed with PTA, which gives an unreactive form confirmed by NMR as N-protonated PTA (acid dissociation constant K(a) = 0.61 M), but in no other cases with [H(+)] in the range 0.30-2.00 M. The narrow spread of rate constants for all but the CO reaction is consistent with an I(d) dissociative interchange mechanism. In addition NMR studies with H(2)(17)O enriched solvent are too slow for direct determination of the water-exchange rate constant indicating a value <10(3) s(-)(1). Equilibrium constants/M(-)(1) for 1:1 complexing with the different X groups at the Ni are obtained for PTA (2040) and TPPTS(3)(-) (8900) by direct spectrophotometry and from kinetic studies (k(f)/k(b)) for Cl(-) (97), Br(-) (150), NCS(-) (690), and CO (5150). No NCS(-) substitution at the Ni is observed in the case of the heterometallic cube [Mo(3)Ni(L)S(4)(H(2)O)(9)](4+), with tridentate 1,4,7-triazacyclononane(L) coordinated to the Ni. Substitution of NCS(-) for H(2)O, at the Mo's of [Mo(3)NiS(4)(H(2)O)(10)](4+) and [Mo(3)(NiL)S(4)(H(2)O)(9)](4+) are much slower secondary processes, with k(f) = 2.7 x 10(-)(4) M(-)(1) s(-)(1) and 0.94 x 10(-)(4) M(-)(1) s(-)(1) respectively. No substitution of H(2)O by TPPTS(3)(-) or CO is observed over approximately 1h at either metal on [Mo(3)FeS(4)(H(2)O)(10)](4+), on [Mo(4)S(4)(H(2)O)(12)](5+) or [Mo(3)S(4)(H(2)O)(9)](4+).