Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal

This article was downloaded by: [Yahia Z. Hamada] On: 23 October 2013, At: 11:37 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lsrt20

Speciation of Molybdenum(VI)-Citric Acid Complexes in Aqueous Solutions a

a

a

Yahia Z. Hamada , Nabil Bayakly , Denisha George & Troy Greer

a

a

Division of Natural and Mathematical Sciences , LeMoyne-Owen College , Memphis, TN Published online: 16 Oct 2008.

To cite this article: Yahia Z. Hamada , Nabil Bayakly , Denisha George & Troy Greer (2008) Speciation of Molybdenum(VI)Citric Acid Complexes in Aqueous Solutions, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 38:8, 664-668 To link to this article: http://dx.doi.org/10.1080/15533170802371323

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 38:664–668, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 1553-3174 print / 1553-3182 online DOI: 10.1080/15533170802371323

Speciation of Molybdenum(VI)-Citric Acid Complexes in Aqueous Solutions Yahia Z. Hamada, Nabil Bayakly, Denisha George, and Troy Greer Division of Natural and Mathematical Sciences, LeMoyne-Owen College, Memphis, TN

Downloaded by [Yahia Z. Hamada] at 11:37 23 October 2013

6+

The chemistry of molybdenum(VI) (Mo ) encounters very complex pathways even when reacting with the simplest of ligands (the aqua ligand). Citric acid (Cit) is considered to be a simple organic ligand. From our efforts to study citric acid with a variety of metal ions (Hamada et al. 2003 [41] and Hamada et al. 2006) a di-hydrolytic complex (or dihydroxo complex) of the Mo6+ : Cit system has been observed using both a speciation computer program and potentiometric titrations in aqueous so◦ lutions at 25 C. The speciation diagrams show that the percentage of formation of this di-hydrolytic complex species overshadows the percentage of formation of the individual free citric acid species. We have taken into account the presence of the following species: the mononuclear species Mo(H−1 Cit), Mo(H−1 Cit)(OH), and Mo(H−1 Cit)(OH)2 , and the di-nuclear Mo2 (H−1 Cit)(OH)2 complex. Among all complexes taken into account, only the dihydrolytic complex Mo(H−1 Cit)(OH)2 has been detected in appreciable percentages. The UV-vis titrations performed at different pH values are in a good agreement with the chemistry literature. Further experimental and theoretical studies are underway in this area. Keywords

3 articles were found.[19–21] When the terms molybdenum and potentiometric or molybdenum and citrate were combined and used, no articles were found. For comparison, a similar literature survey was used within all American Chemical Society journals and, surprisingly, a relatively small number of articles was found.[22–25] These four articles were found by including the terms molybdenum and citrate in both the title and abstract. It is obvious from a thorough literature survey that the investigation herein on the Mo6+ and Cit system in aqueous solutions is merited. Citrate is everywhere in nature. It is involved in the active sites of bacterial metalloenzymes including aconitase, a key enzyme in Krebs cycle.[26] It is also found in human blood plasma at a concentration of about 0.1 mM.[27] Nitrogenase, isolated from N2 -fixing bacteria, catalyzes the reduction of N2 to ammonia according to Eq. 1, where ATP is adenosine triphosphate and Pi is inorganic phosphate. N2 +8H + 8e− + 16MgATP → 2NH3 +H2 + 16MgADP

aqueous solutions, citrate, Mo6+ , potentiometric titrations, speciation diagram

INTRODUCTION A detailed literature survey of the Australian Journal of Chemistry (AJC), as a non-U.S. chemistry journal, indicated a lack of reports regarding the aqueous solution chemistry of the Mo6+ -Cit system. When the term molybdenum was used as the literature survey search term within the AJC, a total of 18 hits were returned.[1–18] These 18 articles were for the chemistry of molybdenum in the following oxidation states: Mo(0), Mo(III), Mo(IV), and Mo(VI). When the terms molybdenum and aqueous solutions were combined and used as search terms, only

Received 6 June 2008; accepted 28 July 2008. This work was supported in part from NSF under Grant #HRD0411493. We also thank many faculty members, especially Dr. S. Painter at the division of Natural and Mathematical Sciences of LeMoyne-Owen College, for reading the manuscript. Address correspondence to Y. Z. Hamada, Division of Natural and Mathematical Sciences, LeMoyne-Owen College, 807 Walker Ave., Memphis, TN 38126. E-mail: Yahia [email protected]

+ 16Pi

[1]

X-ray structural analysis of nitrogenase has revealed the structure of the FeMo cofactor.[28–32] An essential part of the cofactor contains either the Mo-Citrate or the Mo-Homocitrate fragment.[28–32] Tridentate coordination of citrate through the alcoholic, the central carboxyl, and one of the terminal carboxyl groups is the basic feature of monomeric and dimeric citrate complexes.[23] This feature has been seen in the mononuclear K4 [MoO3 (cit)] 2H2 O, the dinuclear K4 [(MoO2 )2 O(Hcit)2 ] 4H2 O, and the following mononuclear complexes: (NH4 )5 [Fe(cit)2 ].2H2 O, (NH4 )5 [Al(cit)2 ].2H2 O, and (NH4 )4 [Ni(cit)2 ].2H2 O.[23,33–35] In all cases, the pH of the medium has been the principle variable controlling complex formation. Complexation of Mo6+ and Cit has been studied by photochemical, potentiometry, NMR (Nuclear Magnetic Resonance), and X-ray crystallography techniques.[22,36–39] These studies found many Mo-Cit complexes mainly as dimmers, tetramers, or higher oligomers.[22,36,39] When monomers were detected in aqueous solutions, they were for the multiprotonated states.[37,38] Such protonated Mo6+ species had the designated nomenclature of

664

665

SPECIATION OF MOLYBDENUM(VI)-CITRIC ACID COMPLEXES

111, 112, 113, 114, 224, 225, 226, 124, 125, etc. for the formula [Mop (Cit)q (H)r ]. The first index in naming the complex (p) stands for the number of the MoO3 fragments. The second index (q) stands for the number of Cit3− as a ligand, and the last index (r) stands for the number of hydrogen ions. Here we are reporting a precise account of the interaction of Mo6+ with Cit in aqueous solutions at 25◦ C using a speciation program and the potentiometric techniques in aqueous solutions under constant temperature and ionic strength. UV-vis titrations are also conducted that are in a good agreement with the literature data.[38]


EXPERIMENTAL Materials and Method All solutions were prepared using doubly deionized (D.I.) water and reagent-grade crystals purchased form Sigma-Aldrich (St. Louis, MO). Citric acid, C6 H8 O7 ·H2 O, formula weight 210.14 g.mol−1 and crystalline ammonium salt of molybdic acid, (NH4 )6 Mo7 O24 ·4H2 O, 83% MoO3 , formula weight 1235.9 g.mol−1 were used as received. The pH values of all solutions were adjusted using sodium hydroxide solution of known concentration (typically ∼0.1 mol·L−1 ). The pH values were measured using the calibrated Orion pH electrode-meter combination model 720A+ (Thermo Electric Corporation), which measures the pH values to the thousandths in 0.1 mol.L−1 ionic strength using the appropriate amounts of NaNO3 solution. The preparation of the potentiometric titration solutions and the exact procedures for conducting the potentiometric titrations have been discussed in detail elsewhere.[40–43] In summary, the potentiometric titration solutions were contained in a 250-mL beaker equipped with a magnetic stirring bar. The beaker was covered with a custom-made Teflon cover. Before each titration, the titration solutions were allowed to stir for 25 min for complete equilibrium. The NaOH titrant was added in 100-µL increments using an Eppendorf micropipette with continuous stirring. The time intervals between the additions of the NaOH solution were set to 5 min, which was sufficient to get each of the pH values stabilized and reach complete equilibrium. All experiments were conducted in 0.1 M NaNO3 solutions as ionic strength adjustor. All UV-vis spectroscopy measurements were conducted using a T60 high-performance spectrophotometer in connection with UVWIN software version 5.0, both purchased from Advanced ChemTech (Louisville, KY). Samples were prepared in D.I. water at 25◦ C. The entire UV-vis spectrum was scanned from 200 to 1100 nm using quartz cuvettes with optical path length of 1 cm. A reference cuvette filled with D.I. water was used with all measurements. The concentration of the metal was = 8.44 × 10−3 mol.L−1 . The UV-vis spectra were collected at the pH values of 4.20, 4.50, 5.00, and 5.77.

FIG. 1. Potentiometric titration curves of all Mo6+ :Citric acid (Cit) system in 1:0, 1:2, 1:4, 1:6, 1:8, and 1:10 moalr ratios. The arrows show the increase of moalr ratio. [MO6+ ] was always set to 5.0 × 10−4 M. 0.1 M NaNO3 , 25◦ C.

of all plots shown in Figure 1. Examination of Figure 1 reveals that the titration of Mo6+ in the absence of Cit released at least eight protons. It appears that when the free citric acid was titrated with NaOH, the three protons on the three carboxylate groups were released with a major inflection at three equivalents exactly (plots are not shown). The release of three protons from the three carboxylates of Cit has been shown previously.[40–42] The alcoholic proton is released only upon metal complexation.[33–40] It is worth mentioning that Mo6+ :Cit did not form any precipitate at any pH value in any molar ratio of all titrations performed. TABLE 1 Potentiometric titration data for the Mo6+ :Cit in different molar ratios. Mo6+ : Cit 1:0 1:2 1:4 1:6 1:8 1:10 a

RESULTS AND DISCUSSION Figure 1 shows the potentiometric titration plots and the exact locations of milliliters of the titrant used. Table 1 is the summary

mL

Eq.a

σb

# Runs

Remarks

3.70 7.70 11.10 15.00 17.70 18.70

8.35 19.70 27.50 34.00 40.00 42.20

0.12 0.71 0.10 0.15 0.10 0.20

7 6 3 3 3 3

No Cit present Hydrolysis Hydrolytic species Hydrolytic species Excess Cit Excess Cit

The number of equivalents (Eq.) is defined as the number of millimoles of titrant per number of millimoles of Mo6+ . b Each experiment has been repeated three times or more. The corresponding standard deviation of all runs is shown in a separate column.


666

HAMADA ET AL.

For the 1:2 titration system it appears that eight protons were released from one mole of Mo6+ plus eight more protons from the two moles of citrate in addition to three hydrolytic protons from the aqua ligands present. The only source of protons to be titrated beyond the eight protons out of the free Mo6+ and the four protons out of each citrate is the aqua ligands that are present in the first hydration sphere of the Mo6+ . It is worth mentioning here that citric acid is commonly known as a tricarboxylic acid; however, the presence of the alcoholic proton make the acid release four protons when reacting with various metal ions.[22,23,33–41] Each subsequent two moles of citrate added release six or seven protons form the free extra moles of Cit. This is apparent from the 1:4, 1:6, 1:8 titrations. When the number of moles of citrate increased to tenfold, the graphs essentially superimposed on one another, indicating that the excess amount of citric acid present did not change the identity of the complexes formed. A closer examination of Figure 1 shows the appearance of two inflection points, one minor and another major. To confirm the exact locations of the inflection points, the first derivatives (slopes) of the observed pH values were plotted versus the number of equivalents of added base. This is shown in Figure 2. Figure 2 is a representative graph for some of the runs performed. The rest of the titrations are available in the supporting material. In this study, the speciation diagrams generated were based on the stability constants taken from the chemistry literature.[44]

These constants were treated with the program HYSS.[45] In this program, pKw of water was set to 13.78 taken from the literature.[46] The rest of the conditions set forth were identical to the conditions set in the experimental setup of the potentiometric titrations shown in Figure 1. Thus, the stability constants were used for the interpretation of the proposed speciation model shown in Figure 3. In this model, it seems that the di-hydroxo complex or Mo(H−1 Cit)(OH)2 is the dominant species over all species that were taken into account. We have set the HYSS program to take into account the following species to be refined simultaneously: the mononuclear species Mo(H−1 Cit), Mo(H−1 Cit)(OH), Mo(H−1 Cit)(OH)2 , and the dinuclear Mo2 (H−1 Cit)(OH)2 complex. As Figure 3 shows, the mononuclear di-hydroxo species Mo(H−1 Cit)(OH)2 not only overshadows the formation of the other Mo6+ species but also the individual species of the free Cit. It is worth mentioning here that the designation (H−1 Cit) stands for the totally deprotonated Cit in which the three carobxylates as well as the alcoholic protons have been removed from Cit.

FIG. 2. Slopes of the observed pH values (pHobs ) vs. the number of equivalents of added base to the Mo6+ :citrate system in 1:2, 1:4, and 1:6 molar ratios. The maximum slopes indicate the position of the major inflection points. Minors are indicated by *.

FIG. 3. Speciation diagram of Mo6+ : Cit in 1:2 molar ratio using the conditions: 0.10 M NaOH and pKw = 13.78. Species taken into account were: the monomers Mo(H−1 Cit), Mo(H−1 Cit)(OH), Mo(H−1 Cit)(OH)2 , and the dimer Mo2 (H−1 Cit) (OH)2 .

CONCLUSION This study revealed that there are a large number of protons released from the Mo6+ :Cit system. Even with the simplest of ligands, H2 O, the titration of the free metal ion released a total of at least eight protons. This is evident from the large number of equivalents of added titrant per Mo6+ to terminate the inflection points in all titration systems. For example, the


SPECIATION OF MOLYBDENUM(VI)-CITRIC ACID COMPLEXES

1:8 and the 1:10 titration systems required 40 and 42 equivalents, respectively, of added titrant (see Table 1 for the definition of the term equivalents). By using speciation software program and the potentiometric techniques in aqueous solutions under constant temperature and ionic strength, we presented a successful speciation model for the appearance of a new dihydrolytic species, namely Mo(H−1 Cit)(OH)2 in aqueous solutions at room temperature. The UV-vis experiments preformed in this study at different pH values confirmed the change in absorption spectra with pH that presented by Cruywagen and others.[38] It is worth mentioning that all photochemical, potentiometry, NMR, and X-ray crystallography studies concerning the Mo6+ :Cit system never described the di-hydroxo complex presented here.[36–39] Attempts to crystallize this species from aqueous solutions yielded a fine powder material not suitable for crystallography. Attempts are underway in our laboratory to crystallize this species from other solvents. Stability constant measurements and potentiometric titrations became more powerful tools when coupled with UV-vis spectroscopy in determining the metal species present in aqueous solutions.[47,48] REFERENCES 1. Budge, J.A., and Broomhead, J.R. Molybdenum and tungsten nitrosyl complexes with dithiocarbamate ligands. Aust. J. Chem., 1979, 32(6), 1187– 1198. 2. Colton, R., and Tomkins, I.B. Carbonyl halides of the group VI transition metals. I. Molybdenum tetracarbonyl dichloride and some of its derivatives. Aust. J. Chem., 1966, 19(7), 1143–1146. 3. Elliott, R.L., Nichols, P.J., and West, B.O. Synthesis of heterobinuclear oxo-bridged compounds of chromium, iron, manganese and molybdenum. Aust. J. Chem., 1986, 39(7), 975–985. 4. Epstein, N.A., Horton, J.L., Vogels, C.M., Taylor, N.J., and Westcott, S.A. Synthesis and characterization of hydrophilic hydroxy-pyridinones and their complexes with molybdenum(VI). Aust. J. Chem., 2000, 53(8), 687–691. 5. Sutton, G.J. Some studies in inorganic complexes. XIII. Chromium(III) and molybdenum(III) with 2-aminomethylpyridine (2-picolylamine). Aust. J. Chem., 1962, 15(2), 232–234. 6. O’Donnell, T.A., Wilson, P.W. Reactivity of transition metal fluorides. V. Reactions of hexafluorides of molybdenum, tungsten, and uranium with ionic chlorides. Aust. J. Chem., 1968, 21(6), 1415–1419. 7. Nagarajan, G. Potential constants for the hexafluorides of molybdenum and rhenium. Aust. J. Chem., 1963, 16(5), 906–907. 8. Bolzan, A.A., Kennedy, B.J., and Howard, C.J. Neutron powder diffraction study of molybdenum and tungsten dioxides. Aust. J. Chem., 1995, 48(8), 1473–1477. 9. Goh, W., Lim, and M. Molybdenum(VI) complexes of Schiff bases derived from salicylaldehyde and 2-aminoethanethiol (cysteamine). Aust. J. Chem., 1984, 37(11), 2235–2242. 10. van Den Bergen, A., Murray, K.S., West, and B.O. Molybdenum(IV) halide complexes with the chelating ligands Schiff bases, acetylacetoae, and 8hydroxyquinoline. Aust. J. Chem., 1972, 25(4), 705–713. 11. Andersonmckay, J., Savage, G.P., and Simpson, G.W. Molybdenum hexacarbonyl promoted ring-opening of hydroxyimino isoxazoles: Unexpected pyrazole formation.Aust. J. Chem., 1996, 49(1), 163–166. 12. Magee, R.J., Liesegang, J., and Stojkovski, S. Molybdenum binding by Pseudomonas aeruginosa., Aust. J. Chem., 1986, 39(8), 1205–1212. 13. Hawkins, C.J., and McEniery, M.L. Circular dichroism spectra of cobalt(III) platinum(II) and molybdenum(0) complexes of 1-phenylethane1,2-diamine. Aust. J. Chem., 1979, 32(7), 1433–1442.

667

14. Colton, R., and Scollary, G.R. Carbonyl halides of the group VI transition metals. X. Further dithiocarbamate derivatives of molybdenum halocarbonyls. Aust. J. Chem., 1968, 21(6), 1427–1424. 15. Steele, M.C. A new complex cyanide of molybdenum. Aust. J. Chem., 1957, 10(4), 404–408. 16. Steele, M.C. A carbonato complex of molybdenum K6 Mo(CO3 )5 .2H2 O. Aust. J. Chem., 1957, 10(3), 367–368. 17. Steele, M.C. An oxalato complex of molybdenum (NH4 )4 Mo(OX)4 .8H2 O. Aust. J. Chem., 1957, 10(3), 368–369. 18. Gagnon, M.K.J., St. Germain, T.R., Vogels, C.M., McNamara, R.A., Taylor, N.J., and Westcott, S.A. Synthesis and hydroboration of lipophilic hydroxypyridinones and their complexes with molybdenum(VI).Aust. J. Chem., 2000, 53(8), 693–697. 19. Belton, P.S., Cox, I.J., and Harris, R.K., O’Connor, M.J. S-33 and O-17 N.M.R-Studies of thiomolybdates and thiotungstates. Aust. J. Chem., 1986, 39(7), 943–952. 20. Gheller, S.F., Sidney, M., Masters, A.F., Brownlee, R.T.C., O’Conner, M.J., and Wedd, A.G. Applications of molybdenum-95 N.M.R. spectroscopy. X. Polyoxomolybdates. Aust. J. Chem., 1984, 37(9), 1825–1832. 21. Dunne, S.J., Burns, R.C., Hambley, T.W., and Lawrance, G.A. Oxidation of manganese(II) by peroxomonosulfuric acid in aqueous solution in the presence of molybdate. Crystal-structure of the K6 [MnMo9 O32 ].6H2 O product. Aust. J. Chem., 1992, 45(4), 685–693. 22. Zhou, Z.H., Deng, Y.F., Cao, Z.X., Zhang, R.H., and Chow, Y.L. Dimeric dioxomolybdenum(VI) and oxomolybdenum (V) complexes with citrate at very low pH and neutral conditions. Inorg. Chem., 2005, 44(20), 6912– 6914. 23. Zhou, Z.H., Wan, H.L., and Tsai, K.R. Synthesis and spectroscopic and structural characterization of molybdenum(VI) citrato monomeric raceme and dimer, K4 [MoO3 (cit)] 2H2 O, the dinuclear K4 [(MoO2 )2 O(Hcit)2 ] 4H2 O. Inorg. Chem., 2000, 39(1), 59–64. 24. Takuma, M., Ohki, Y., and Tatsumi, K. Molybdenum carbonyl complexes with citrate and its relevant carboxylates. Organometallics 2005, 24(6), 1344–1347. 25. Bray, R.C., Adams, B., Smith, A.T., Bennett, B., and Bailey, S. Reversible dissociation of thiolate ligands from molybdenum in an enzyme sulfoxide reductase family. Biochemistry 2000, 39(37), 11258–11269. 26. Cowan, J.A. In Inorganic Biochemistry/Fundamentals of Inorganic Biochemistry an Introduction; Wiley-VCH: Hoboken, NJ, 1997. pp. 190–199. 27. Martin, B.R. Citrate binding of Al3+ and Fe3+ . J. Inorg. Biochem., 1986, 28, 181–187. 28. Kim, J., and Rees, D.C. Structural models for the metal centers in the nitrogenase molybdenum-iron protein. Science 1992, 257, 1677–1682. 29. Chan, M.K., Kim, J., and Rees, D.C. The nitrogenase Fe-Mo-cofactor and P-cluster pair. 2.2 A˚ resolution structures. Science 1993, 260, 792–794. 30. Kim, J., and Rees, D.C. Nitrogenase and biological nitrogen fixation. Biochemistry 1994, 33, 389–398. 31. Kim, J., and Rees, D.C. Crystallographic structure and functional implication of the nitrogenase molybdenum-iron protein from A. Vinelandii. Nature 1992, 360, 553–560. 32. Kim, J., Wood, D., and Rees, D.C. X-ray crystal structure of the nitrogenase molybdenum-iron protein from Clostridium pasteurianum at 3.0-A˚ resolution. Biochemistry 1993, 32(28), 7104–7115. 33. Matzapetakis, M., Raptopoulou, C.P., Tsohos, A., Papaefthymiou, V., Moon, N., and Salifoglou, A. Synthesis, spectroscopic and structural characterization of the first mononuclear, water soluble iron-citrate complex, (NH4 )5 Fe(cit)2 2H2 O. J. Am. Chem. Soc., 1998, 120, 13266–13267. 34. Matzapetakis, M., Raptopoulou, C.P., Terzis, A., Lakatos, A., Kiss, and T., Salifoglou, A.Synthesis, spectroscopic and structural characterization of the first mononuclear, water soluble aluminum-citrate complex, (NH4 )5 Al(cit)2 2H2 O. Inorg. Chem., 1999, 38, 618–619. 35. Zhou, Z.H., Lin, Y.J., Zhang, H.B., Lin, G.D., and Tsai, K.R. Synthesis, structure, and spectroscopic properties of nickel(II) citrato complexes, (NH4 )2 [Ni(Hcit) (H2 O)2 ]2 .2H2 O and (NH4 )4 [Ni(Hcit)2 ].2H2 O. J. Coord. Chem., 1997, 42, 131–141.


668

HAMADA ET AL.

36. Pedrosa de Jesus, J.D., Farropas, M. de D., O’Brien, P., Gillard, R.D., and Williams, P.A. Photochemical studies of Mo(VI) citric acid complex Mo2 O5 OH(H2 O)(C6 H5 O7 )2− . Transit. Met. Chem., 1983, 8, 193–195. 37. Cruywagen, J.J., and Van de Water, R.F. Complexation between molybdenum and citrate: A potentiometric and calorimetric investigation. Polyhedron 1986, 5, 521–526. 38. Cruywagen, J.J., Rohwer, E.A., and Wessels, G.F.S. Molybdenum(VI) complex formation-8. Equilibria and thermodynamic quantities for the reaction with citrate. Polyhedron 1995, 14, 3481–3493. 39. Nassimbeni, L.R., Niven, M.L., Cruywagen, J.J., and Heyns, J.B. Complexation between molybdenum(VI) and citrate: Crystal and molecular structure of [Mo4 O11 (citrate)2 ](Me3 N(CH2 )6 NMe3 )2 . 12H2 O. J. Chem. Crystallogr., 1987, 17, 373–382. 40. Hamada, Y.Z., Zhepeng, W., and Harris, W.R. Competition between transferrin and serum ligands citrate and phosphate for the binding of aluminum. Inorg. Chem., 2003, 42, 3262–3273. 41. Hamada, Y.Z., Carlson, B.L., and Shank, J.T. Potentiometric and UV-vis spectroscopy studies of citrate with the hexaquo Fe3+ and Cr3+ metal ions. Syn. Reac. Inorg. Metal-Org. Chem., 2003, 33(8), 1425–1440.

42. Hamada, Y.Z., Bayakly, N., Peipho, A., and Carlson, B.L. Accurate potentiometric studies of chromium-citrate and ferric-citrate complexes in aqueous solutions at physiological and alkaline pH values. Syn. Reac. Inorg. Metal-Org. and Nano-Metal Chem., 2006, 36, 469–476. 43. Hamada,Y.Z., Carlson, B.L., and Dangberg, J. Interaction of malate and lactate with chromium(III) and iron(III) in aqueous solutions. Syn. Reac. Inorg. Metal-Org. and Nano-Metal Chem., 2005, 35, 515–522. 44. Martell, A.E., Smith, R.M., and Motekaitis, R.J. Critical Stability Constants Database, Version 6.0; NIST, Texas A & M University: College Station, TX, 2001. 45. Alderighi, L., Gans, P., Ienco, A., Perters, D., Sabatini, A., and Vacca, A. Hyperquad simulation and speciation (Hyss): a utility program for the investigation of equilibria involving soluble and partially soluble species. Coord. Chem. Rev., 1999, 184, 311–318. 46. Sweeton, F.H., Mesmer, R.E., and Baes, C.F., Jr. Acidity measurements at elevated temperature. VII. Dissociation of water. J. Solution Chem., 1974, 3, 191–214. 47. Kettle, S.F. A. Physical Inorganic Chemistry, A Coordination Chemistry Approach, Spektrum; University Science Book: Sausalito, CA, 1996. 48. Skoog, D.A., West, D.M., Holler, F.J., and Crouch, S.R. Fundamentals of Analytical Chemistry, 8th ed.; Brooks-Cole/Thomson: Belmont, CA, 2004.