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Metal aquo complex

February 15, 2024

In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand ("homoleptic aquo complexes"), but of course many complexes are known to consist of a mix of aquo and other ligands.[1][2]

Stoichiometry and structure edit

Hexa-aquo complexes edit

 
Structure of an octahedral metal aquo complex.
 
Chromium(II) ion in aqueous solution.

Most aquo complexes are mono-nuclear, with the general formula [M(H2O)6]n+, with n = 2 or 3; they have an octahedral structure. The water molecules function as Lewis bases, donating a pair of electrons to the metal ion and forming a dative covalent bond with it. Typical examples are listed in the following table.

Complex colour electron config. M−O distance (Å)[3] water exchange
rate (s−1, 25 °C)[4]		 M2+/3+ self-exchange
rate (M−1s−1, 25 °C)
[Ti(H2O)6]3+ violet (t2g)1 2.025 1.8×105 —
[V(H2O)6]2+ violet (t2g)3 2.12 8.7×101 fast
[V(H2O)6]3+ green (t2g)2 1.991[5] 5.0×102 fast
[Cr(H2O)6]2+ blue (t2g)3(eg)1 2.06 and 2.33 1.2×108 slow
[Cr(H2O)6]3+ violet (t2g)3 1.961 2.4×10−6 slow
[Mn(H2O)6]2+ pale pink (t2g)3(eg)2 2.177 2.1×107 —
[Fe(H2O)6]2+ pale blue-green (t2g)4(eg)2 2.095 4.4×106 fast
[Fe(H2O)6]3+ pale violet (t2g)3(eg)2 1.990 1.6×102 fast[6]
[Co(H2O)6]2+ pink (t2g)5(eg)2 2.08 3.2×106 —
[Ni(H2O)6]2+ green (t2g)6(eg)2 2.05 3.2×104 —
[Cu(H2O)6]2+ blue (t2g)6(eg)3 1.97 and 2.30 5.7×109 —
[Zn(H2O)6]2+ colorless (t2g)6(eg)4 2.03-2.10 fast —

Tutton's salts are crystalline compounds with the generic formula (NH4)2M(SO4)2·(H2O)6 (where M = V2+, Cr2+, Mn2+, Co2+, Ni2+, or Cu2+). Alums, MM′(SO4)2(H2O)12, are also double salts. Both sets of salts contain hexa-aquo metal cations.

Tetra-aquo complexes edit

Silver(I) forms [Ag(H2O)4]+, a rare example of a tetrahedral aquo complex.[7] Palladium(II) and platinum(II) were once thought to form square planar aquo complexes.[8]

Octa- and nona- aquo complexes edit

Aquo complexes of lanthanide(III) ions are eight- and nine-coordinate, reflecting the large size of the metal centres.

Binuclear-aquo complexes edit

 
Structure of [Co2(OH2)10]4+ color code: red = O, white = H, blue = Co.

In the binuclear ion [Co2(OH2)10]4+ each bridging water molecule donates one pair of electrons to one cobalt ion and another pair to the other cobalt ion. The Co-O (bridging) bond lengths are 213 picometers, and the Co-O (terminal) bond lengths are 10 pm shorter.[9]

The complexes [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+ contain metal-metal bonds.[7]

Hydroxo- and oxo- complexes of aquo ions edit

Monomeric aquo complexes of Nb, Ta, Mo, W, Mn, Tc, Re, and Os in oxidation states +4 to +7 have not been reported.[8] For example, [Ti(H2O)6]4+ is unknown: the hydrolyzed species [Ti(OH)2(H2O)n]2+ is the principal species in dilute solutions.[10] With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo-complexes.

Aquo complexes of the lanthanide cations edit

Lanthanide salts often or perhaps characteristically form aquo complexes. The homoleptic tricationic aquo complexes have nine water ligands.[11]

Reactions edit

Some reactions considered fundamental to the behavior of metal aquo ions are ligand exchange, electron-transfer, and acid-base reactions.

Water exchange edit

Ligand exchange involves replacement of a water ligand ("coordinated water") with water in solution ("bulk water"). Often the process is represented using labeled water H2O·:

 

In the absence of isotopic labeling, the reaction is degenerate, meaning that the free energy change is zero. Rates vary over many orders of magnitude. The main factor affecting rates is charge: highly charged metal aquo cations exchange their water more slowly than singly charged cations. Thus, the exchange rates for [Na(H2O)6]+ and [Al(H2O)6]3+ differ by a factor of 109. Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for [Al(H2O)6]3+ and [Ir(H2O)6]3+ differ by a factor of 109 also.[4] Water exchange usually follows a dissociative substitution pathway, so the rate constants indicate first order reactions.

Electron exchange edit

This reaction usually applies to the interconversion of di- and trivalent metal ions, which involves the exchange of only one electron. The process is called self-exchange, meaning that the ion appears to exchange electrons with itself. The standard electrode potential for the following equilibrium:

[M(H2O)6]2+ + [M'(H2O)6]3+ ⇌ [M(H2O)6]3+ + [M'(H2O)6]2+
Standard redox potential for the couple M2+, M3+ (V)
V Cr Mn Fe Co
−0.26 −0.41 +1.51 +0.77 +1.82

shows the increasing stability of the lower oxidation state as atomic number increases. The very large value for the manganese couple is a consequence of the fact that octahedral manganese(II) has zero crystal field stabilization energy (CFSE) but manganese(III) has 3 units of CFSE.[12]

Using labels to keep track of the metals, the self-exchange process is written as:

 

The rates of electron exchange vary widely, the variations being attributable to differing reorganization energies: when the 2+ and 3+ ions differ widely in structure, the rates tend to be slow.[13] The electron transfer reaction proceeds via an outer sphere electron transfer. Most often large reorganizational energies are associated with changes in the population of the eg level, at least for octahedral complexes.

Acid–base reactions edit

Solutions of metal aquo complexes are acidic owing to the ionization of protons from the water ligands. In dilute solution chromium(III) aquo complex has a pKa of about 4.3:

[Cr(H2O)6]3+ ⇌ [Cr(H2O)5(OH)]2+ + H+

Thus, the aquo ion is a weak acid, of comparable strength to acetic acid (pKa of about 4.8). This pKa is typical of the trivalent ions. The influence of the electronic configuration on acidity is shown by the fact that [Ru(H2O)6]3+ (pKa = 2.7) is more acidic than [Rh(H2O)6]3+ (pKa = 4), despite the fact that Rh(III) is expected to be more electronegative. This effect is related to the stabilization of the pi-donor hydroxide ligand by the (t2g)5 Ru(III) centre.[7]

In concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation, to form polymeric species. Many minerals are assumed to form via olation. Aquo ions of divalent metal ions are less acidic than those of trivalent cations.

The hydrolyzed species often exhibit very different properties from the precursor hexaaquo complex. For example, water exchange in [Al(H2O)5OH]2+ is 20000 times faster than in [Al(H2O)6]3+.

See also edit

References edit

  1. ^ Mark I. Ogden and Paul D. Beer "Water & O-Donor Ligands" in Encyclopedia of Inorganic Chemistry, Wiley-VCH, 2006, Weinheim. doi:10.1002/0470862106.ia255
  2. ^ Lincoln, S.F.; Richens, D.T.; Sykes, A.G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN 9780080437484.
  3. ^ For Mn(II), Fe(II), Fe(III):Sham, T. K.; Hastings, J. B.; Perlman, M. L. (1980). "Structure and Dynamic Behavior of Transition-Metal Ions in Aqueous Aolution: an EXAFS Study of Electron-Exchange Reactions". J. Am. Chem. Soc. 102 (18): 5904–5906. doi:10.1021/ja00538a033.. For Ti(III), V(III), Cr(III): Kallies, B.; Meier, R. (2001). "Electronic Structure of 3d [M(H2O)6]3+ Ions from ScIII to FeIII: A Quantum Mechanical Study Based on DFT Computations and Natural Bond Orbital Analyses". Inorg. Chem. 40 (13): 3101–3112. doi:10.1021/ic001258t. PMID 11399179.
  4. ^ a b Helm, Lothar; Merbach, André E. (2005). "Inorganic and Bioinorganic Solvent Exchange Mechanisms". Chemical Reviews. 105 (6): 1923–1959. doi:10.1021/cr030726o. PMID 15941206.
  5. ^ Cotton, F. A.; Fair, C. K.; Lewis, G. E.; Mott, G. N.; Ross, F. K.; Schultz, A. J.; Williams, J. M. (1984). "Precise Structural Characterizations of the Hexaaquovanadium(III) and Diaquohydrogen Ions. X-ray and Neutron Diffraction Studies of [V(H2O)6][H5O2](CF3SO3)4". Journal of the American Chemical Society. 106 (18): 5319–5323. doi:10.1021/ja00330a047.
  6. ^ Grant, M.; Jordan, R. B. (1981). "Kinetics of Solvent Water Exchange on Iron(III)". Inorganic Chemistry. 20: 55–60. doi:10.1021/ic50215a014.
  7. ^ a b c Lincoln, S. F.; Richens, D. T.; Sykes, A. G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. Comprehensive Coordination Chemistry II. Vol. 1. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN 9780080437484.
  8. ^ a b Persson, Ingmar (2010). "Hydrated Metal Ions in Aqueous Solution: How Regular are Their Structures?". Pure and Applied Chemistry. 82 (10): 1901–1917. doi:10.1351/PAC-CON-09-10-22.
  9. ^ Han, Yin-Feng; Li, Min; Wang, Tian-Wei; Li, Yi-Zhi; Shen, Zhen; Song, You; You, Xiao-Zeng (2008). "A Novel Microporous Hydrogen-Bonding Framework Constructed with Tetrathiafulvalene Tetracarboxylate Ligand: Synthesis, Structure and Magnetic Properties". Inorganic Chemistry Communications. 11 (9): 945–947. doi:10.1016/j.inoche.2008.04.028.
  10. ^ Baes, C.F.; Mesmer, R.E. The Hydrolysis of Cations, (1976), Wiley, New York
  11. ^ Parker, David; Dickins, Rachel S.; Puschmann, Horst; Crossland, Clare; Howard, Judith A. K. (2002). "Being Excited by Lanthanide Coordination Complexes: Aqua Species, Chirality, Excited-State Chemistry, and Exchange Dynamics". Chemical Reviews. 102 (6): 1977–2010. doi:10.1021/cr010452+. PMID 12059260.
  12. ^ Burgess, John (1978). Metal Ions in Solution. Chichester: Ellis Horwood. ISBN 0-85312-027-7. p. 236.
  13. ^ Wilkins, R. G. (1991). Kinetics and Mechanism of Reactions of Transition Metal Complexes (2 ed.). Weinheim: VCH. ISBN 1-56081-125-0.

February 15, 2024
metal, aquo, complex, chemistry, metal, aquo, complexes, coordination, compounds, containing, metal, ions, with, only, water, ligand, these, complexes, predominant, species, aqueous, solutions, many, metal, salts, such, metal, nitrates, sulfates, perchlorates,. In chemistry metal aquo complexes are coordination compounds containing metal ions with only water as a ligand These complexes are the predominant species in aqueous solutions of many metal salts such as metal nitrates sulfates and perchlorates They have the general stoichiometry M H2O n z Their behavior underpins many aspects of environmental biological and industrial chemistry This article focuses on complexes where water is the only ligand homoleptic aquo complexes but of course many complexes are known to consist of a mix of aquo and other ligands 1 2 Contents 1 Stoichiometry and structure 1 1 Hexa aquo complexes 1 2 Tetra aquo complexes 1 3 Octa and nona aquo complexes 1 4 Binuclear aquo complexes 1 5 Hydroxo and oxo complexes of aquo ions 1 6 Aquo complexes of the lanthanide cations 2 Reactions 2 1 Water exchange 2 2 Electron exchange 2 3 Acid base reactions 3 See also 4 ReferencesStoichiometry and structure editHexa aquo complexes edit nbsp Structure of an octahedral metal aquo complex nbsp Chromium II ion in aqueous solution Most aquo complexes are mono nuclear with the general formula M H2O 6 n with n 2 or 3 they have an octahedral structure The water molecules function as Lewis bases donating a pair of electrons to the metal ion and forming a dative covalent bond with it Typical examples are listed in the following table Complex colour electron config M O distance A 3 water exchangerate s 1 25 C 4 M2 3 self exchangerate M 1s 1 25 C Ti H2O 6 3 violet t2g 1 2 025 1 8 105 V H2O 6 2 violet t2g 3 2 12 8 7 101 fast V H2O 6 3 green t2g 2 1 991 5 5 0 102 fast Cr H2O 6 2 blue t2g 3 eg 1 2 06 and 2 33 1 2 108 slow Cr H2O 6 3 violet t2g 3 1 961 2 4 10 6 slow Mn H2O 6 2 pale pink t2g 3 eg 2 2 177 2 1 107 Fe H2O 6 2 pale blue green t2g 4 eg 2 2 095 4 4 106 fast Fe H2O 6 3 pale violet t2g 3 eg 2 1 990 1 6 102 fast 6 Co H2O 6 2 pink t2g 5 eg 2 2 08 3 2 106 Ni H2O 6 2 green t2g 6 eg 2 2 05 3 2 104 Cu H2O 6 2 blue t2g 6 eg 3 1 97 and 2 30 5 7 109 Zn H2O 6 2 colorless t2g 6 eg 4 2 03 2 10 fast Tutton s salts are crystalline compounds with the generic formula NH4 2M SO4 2 H2O 6 where M V2 Cr2 Mn2 Co2 Ni2 or Cu2 Alums MM SO4 2 H2O 12 are also double salts Both sets of salts contain hexa aquo metal cations Tetra aquo complexes edit Silver I forms Ag H2O 4 a rare example of a tetrahedral aquo complex 7 Palladium II and platinum II were once thought to form square planar aquo complexes 8 Octa and nona aquo complexes edit Aquo complexes of lanthanide III ions are eight and nine coordinate reflecting the large size of the metal centres Binuclear aquo complexes edit nbsp Structure of Co2 OH2 10 4 color code red O white H blue Co In the binuclear ion Co2 OH2 10 4 each bridging water molecule donates one pair of electrons to one cobalt ion and another pair to the other cobalt ion The Co O bridging bond lengths are 213 picometers and the Co O terminal bond lengths are 10 pm shorter 9 The complexes Mo2 H2O 8 4 and Rh2 H2O 10 4 contain metal metal bonds 7 Hydroxo and oxo complexes of aquo ions edit Monomeric aquo complexes of Nb Ta Mo W Mn Tc Re and Os in oxidation states 4 to 7 have not been reported 8 For example Ti H2O 6 4 is unknown the hydrolyzed species Ti OH 2 H2O n 2 is the principal species in dilute solutions 10 With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo complexes Aquo complexes of the lanthanide cations edit Lanthanide salts often or perhaps characteristically form aquo complexes The homoleptic tricationic aquo complexes have nine water ligands 11 Reactions editSome reactions considered fundamental to the behavior of metal aquo ions are ligand exchange electron transfer and acid base reactions Water exchange edit Ligand exchange involves replacement of a water ligand coordinated water with water in solution bulk water Often the process is represented using labeled water H2O M H 2 O n z H 2 O M H 2 O n 1 H 2 O z H 2 O displaystyle ce M H2O mathit n mathit z ce H2O star longrightarrow ce M H2O mathit n 1 H2O star mathit z ce H2O nbsp In the absence of isotopic labeling the reaction is degenerate meaning that the free energy change is zero Rates vary over many orders of magnitude The main factor affecting rates is charge highly charged metal aquo cations exchange their water more slowly than singly charged cations Thus the exchange rates for Na H2O 6 and Al H2O 6 3 differ by a factor of 109 Electron configuration is also a major factor illustrated by the fact that the rates of water exchange for Al H2O 6 3 and Ir H2O 6 3 differ by a factor of 109 also 4 Water exchange usually follows a dissociative substitution pathway so the rate constants indicate first order reactions Electron exchange edit This reaction usually applies to the interconversion of di and trivalent metal ions which involves the exchange of only one electron The process is called self exchange meaning that the ion appears to exchange electrons with itself The standard electrode potential for the following equilibrium M H2O 6 2 M H2O 6 3 M H2O 6 3 M H2O 6 2 Standard redox potential for the couple M2 M3 V V Cr Mn Fe Co 0 26 0 41 1 51 0 77 1 82shows the increasing stability of the lower oxidation state as atomic number increases The very large value for the manganese couple is a consequence of the fact that octahedral manganese II has zero crystal field stabilization energy CFSE but manganese III has 3 units of CFSE 12 Using labels to keep track of the metals the self exchange process is written as M H 2 O 6 2 M H 2 O 6 3 M H 2 O 6 3 M H 2 O 6 2 displaystyle ce M H2O 6 2 ce M star H2O 6 3 longrightarrow ce M star H2O 6 3 ce M H2O 6 2 nbsp The rates of electron exchange vary widely the variations being attributable to differing reorganization energies when the 2 and 3 ions differ widely in structure the rates tend to be slow 13 The electron transfer reaction proceeds via an outer sphere electron transfer Most often large reorganizational energies are associated with changes in the population of the eg level at least for octahedral complexes Acid base reactions edit Solutions of metal aquo complexes are acidic owing to the ionization of protons from the water ligands In dilute solution chromium III aquo complex has a pKa of about 4 3 Cr H2O 6 3 Cr H2O 5 OH 2 H Thus the aquo ion is a weak acid of comparable strength to acetic acid pKa of about 4 8 This pKa is typical of the trivalent ions The influence of the electronic configuration on acidity is shown by the fact that Ru H2O 6 3 pKa 2 7 is more acidic than Rh H2O 6 3 pKa 4 despite the fact that Rh III is expected to be more electronegative This effect is related to the stabilization of the pi donor hydroxide ligand by the t2g 5 Ru III centre 7 In concentrated solutions some metal hydroxo complexes undergo condensation reactions known as olation to form polymeric species Many minerals are assumed to form via olation Aquo ions of divalent metal ions are less acidic than those of trivalent cations The hydrolyzed species often exhibit very different properties from the precursor hexaaquo complex For example water exchange in Al H2O 5OH 2 is 20000 times faster than in Al H2O 6 3 See also editHydration number Ligand field theory Metal ammine complex Metal ions in aqueous solutionReferences edit Mark I Ogden and Paul D Beer Water amp O Donor Ligands in Encyclopedia of Inorganic Chemistry Wiley VCH 2006 Weinheim doi 10 1002 0470862106 ia255 Lincoln S F Richens D T Sykes A G 2003 Metal Aqua Ions Comprehensive Coordination Chemistry II pp 515 555 doi 10 1016 B0 08 043748 6 01055 0 ISBN 9780080437484 For Mn II Fe II Fe III Sham T K Hastings J B Perlman M L 1980 Structure and Dynamic Behavior of Transition Metal Ions in Aqueous Aolution an EXAFS Study of Electron Exchange Reactions J Am Chem Soc 102 18 5904 5906 doi 10 1021 ja00538a033 For Ti III V III Cr III Kallies B Meier R 2001 Electronic Structure of 3d M H2O 6 3 Ions from ScIII to FeIII A Quantum Mechanical Study Based on DFT Computations and Natural Bond Orbital Analyses Inorg Chem 40 13 3101 3112 doi 10 1021 ic001258t PMID 11399179 a b Helm Lothar Merbach Andre E 2005 Inorganic and Bioinorganic Solvent Exchange Mechanisms Chemical Reviews 105 6 1923 1959 doi 10 1021 cr030726o PMID 15941206 Cotton F A Fair C K Lewis G E Mott G N Ross F K Schultz A J Williams J M 1984 Precise Structural Characterizations of the Hexaaquovanadium III and Diaquohydrogen Ions X ray and Neutron Diffraction Studies of V H2O 6 H5O2 CF3SO3 4 Journal of the American Chemical Society 106 18 5319 5323 doi 10 1021 ja00330a047 Grant M Jordan R B 1981 Kinetics of Solvent Water Exchange on Iron III Inorganic Chemistry 20 55 60 doi 10 1021 ic50215a014 a b c Lincoln S F Richens D T Sykes A G 2003 Metal Aqua Ions Comprehensive Coordination Chemistry II Comprehensive Coordination Chemistry II Vol 1 pp 515 555 doi 10 1016 B0 08 043748 6 01055 0 ISBN 9780080437484 a b Persson Ingmar 2010 Hydrated Metal Ions in Aqueous Solution How Regular are Their Structures Pure and Applied Chemistry 82 10 1901 1917 doi 10 1351 PAC CON 09 10 22 Han Yin Feng Li Min Wang Tian Wei Li Yi Zhi Shen Zhen Song You You Xiao Zeng 2008 A Novel Microporous Hydrogen Bonding Framework Constructed with Tetrathiafulvalene Tetracarboxylate Ligand Synthesis Structure and Magnetic Properties Inorganic Chemistry Communications 11 9 945 947 doi 10 1016 j inoche 2008 04 028 Baes C F Mesmer R E The Hydrolysis of Cations 1976 Wiley New York Parker David Dickins Rachel S Puschmann Horst Crossland Clare Howard Judith A K 2002 Being Excited by Lanthanide Coordination Complexes Aqua Species Chirality Excited State Chemistry and Exchange Dynamics Chemical Reviews 102 6 1977 2010 doi 10 1021 cr010452 PMID 12059260 Burgess John 1978 Metal Ions in Solution Chichester Ellis Horwood ISBN 0 85312 027 7 p 236 Wilkins R G 1991 Kinetics and Mechanism of Reactions of Transition Metal Complexes 2 ed Weinheim VCH ISBN 1 56081 125 0 Retrieved from https en wikipedia org w index php title Metal aquo complex amp oldid 1158425612, wikipedia, wiki, book, books, library,

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