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| '''Transition metal hydroxide complexes''' are |
'''Transition metal hydroxide complexes''' are ]es containing one or more ] (OH<sup>-</sup>) ]s. The inventory is very large. | ||
| ==Hydroxide as a ligand== | |||
| Hydroxide is classified as an X ligand in the ]. In the ], it is a one-electron ligand when terminal and a three-electron ligand when doubly bridging. | |||
| From the electric structure perspective, hydroxide is a strong pi-donor ligand, akin to fluoride. One consequence is that few polyhydroxide complexes are low spin. Another consequence is that electron-precise hydroxide complexes tend to be rather nucleophilic.<ref name=Nolan>{{cite journal |doi=10.1016/j.ccr.2017.10.012 |title=Hydroxide complexes of the late transition metals: Organometallic chemistry and catalysis |date=2017 |last1=Nelson |first1=David J. |last2=Nolan |first2=Steven P. |journal=Coordination Chemistry Reviews |volume=353 |pages=278–294 |url=https://strathprints.strath.ac.uk/62176/1/Nelson_Nolan_CCR2017_Hydroxide_complexes_of_the_late_transition_metals.pdf }}</ref><ref>{{cite journal |doi=10.1021/ar000132x |title=Formation, Reactivity, and Properties of Nondative Late Transition Metal−Oxygen and −Nitrogen Bonds |date=2002 |last1=Fulton |first1=J. Robin |last2=Holland |first2=Andrew W. |last3=Fox |first3=Daniel J. |last4=Bergman |first4=Robert G. |journal=Accounts of Chemical Research |volume=35 |issue=1 |pages=44–56 |pmid=11790088 |pmc=1473979 }}</ref> | |||
| ==Representative complexes== | ==Representative complexes== | ||
| Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.<ref name=Nolan/> | |||
| ===Homoleptic complexes=== | ===Homoleptic complexes=== | ||
| Only a few homoleptic hydroxide complexes are known |
Only a few homoleptic hydroxide complexes are known. These include the d<sup>6</sup> species {{chem2|(2-)}}<ref>{{cite journal |doi=10.1107/S0567740882006311 |title=Ammonium hexahydroxoplatinat(IV) und Strukturverfeinerung für Kalium-hexahydroxoplatinat(IV) |date=1982 |last1=Bandel |first1=G. |last2=Platte |first2=C. |last3=Trömel |first3=M. |journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry |volume=38 |issue=5 |pages=1544–1546 |bibcode=1982AcCrB..38.1544B }}</ref> and the d<sup>0</sup> complexes {{chem2|(2-) and (2-)}}.<ref>{{cite journal |doi=10.1021/cm101490w |title=Hydroxometalates from Anion Exchange Reactions of [BF<sub>4</sub>]<sup>−</sup> based Ionic Liquids: Formation of [M(OH)<sub>6</sub>)]<sup>2−</sup> (M = Ti, Zr) and [Zr(OH)<sub>5</sub>]<sup>−</sup> |date=2010 |last1=Lin |first1=Hechun |last2=De Oliveira |first2=Peter W. |last3=Huch |first3=Volker |last4=Veith |first4=Michael |journal=Chemistry of Materials |volume=22 |issue=24 |pages=6518–6523 }}</ref> | ||
| ===Mixed ligand complexes=== | ===Mixed ligand complexes=== | ||
| Many complexes are known where hydroxide shares the coordination sphere with other ligands. One pair of such complexes are {<sub>2</sub>(mu-OH)<sub>3</sub>}<sup>3+</sup> and its derivative {<sub>2</sub>(mu-OH)<sub>2</sub>}<sup>4+</sup>.<ref>{{cite |
Many complexes are known where hydroxide shares the coordination sphere with other ligands. One pair of such complexes are {<sub>2</sub>(mu-OH)<sub>3</sub>}<sup>3+</sup> and its derivative {<sub>2</sub>(mu-OH)<sub>2</sub>}<sup>4+</sup>.<ref>{{cite book |doi=10.1002/9780470132548.ch21 |title=μ-Carboxylatodi-μ-Hydroxo-Bis[Triamminecobalt(III)] Complexes |series=Inorganic Syntheses |year=1985 |last1=Wieghardt |first1=K. |last2=Siebert |first2=H. |volume=23 |pages=107–116|isbn=978-0-471-81873-1 }}</ref> | ||
| ] | |||
| ==Reactions== | ==Reactions== | ||
| Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives ]es: | Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives ]es: | ||
| :{{chem2|L_{n}M\sOH + H+ <-> L_{n}M\sOH2+}} | :{{chem2|L_{n}M\sOH + H+ <-> L_{n}M\sOH2+}} where {{chem2|L_{n} }} is the ligand complement on the metal M | ||
| Thus, aquo ligand is a ], of comparable strength to ] (p''K''<sub>a</sub> of about 4.8).<ref name=Lincoln>{{cite book|first1=S. F. |last1=Lincoln|first2=D. T.|last2=Richens|first3=A. G.|last3=Sykes|title=Comprehensive Coordination Chemistry II|chapter=Metal Aqua Ions| |
Thus, aquo ligand is a ], of comparable strength to ] (p''K''<sub>a</sub> of about 4.8).<ref name=Lincoln>{{cite book|first1=S. F. |last1=Lincoln|first2=D. T.|last2=Richens|first3=A. G.|last3=Sykes|title=Comprehensive Coordination Chemistry II|chapter=Metal Aqua Ions|year=2003|volume=1|pages=515–555|doi=10.1016/B0-08-043748-6/01055-0|isbn=9780080437484}}</ref> | ||
| |year=2003|volume=1|pages=515–555|doi=10.1016/B0-08-043748-6/01055-0|isbn=9780080437484}}</ref> | |||
| In principle but not very commonly, metal hydroxides undergo deprotonation, yielding ]es: | In principle but not very commonly, metal hydroxides undergo deprotonation, yielding ]es: | ||
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| Characteristically, hydroxide ligands are compact and basic. They tend to function as ]s. One manifestation of this property is the preponderance of di-and polymetallic hydroxide complexes. A practical consequence of this feature is the tendency of metal aquo complexes to form precipitates of meta hydroxides. | Characteristically, hydroxide ligands are compact and basic. They tend to function as ]s. One manifestation of this property is the preponderance of di-and polymetallic hydroxide complexes. A practical consequence of this feature is the tendency of metal aquo complexes to form precipitates of meta hydroxides. | ||
| ===Bioinorganic chemistry=== | |||
| The hydroxo ligand is a nucleophile. This behavior is relevant to their role in enzymology. In ], a zinc hydroxide binds ]: | |||
| :] | |||
| ]s, ]s responsible for ] (O<sub>2</sub>) transport in some animals have an diiron hydroxide active site. The hydroxide ligand engages the bound O2 through ]. | |||
| The nucleophilicity of hydroxo ligands is relevant to the role of some M-OH centers in enzymology. For example, in ], a zinc hydroxide binds ]:<ref>{{cite journal |doi=10.1021/cr0206263 |title=Synthetic Analogues Relevant to the Structure and Function of Zinc Enzymes |date=2004 |last1=Parkin |first1=Gerard |journal=Chemical Reviews |volume=104 |issue=2 |pages=699–768 |pmid=14871139 }}</ref> | |||
| :{{chem2|L_{n}M\sOH + CO2 <-> L_{n}MO\sO\sCO2H}} | :{{chem2|L_{n}M\sOH + CO2 <-> L_{n}MO\sO\sCO2H}} | ||
| The ] (OEC) consists of a Mn-Ca-O-OH cluster that is responsible for the biosynthesis of O<sub>2</sub>. It is proposed that the O-O bond forming step involves a hydroxide ligand. | |||
| ]s catalyze the hydrolysis peptide bond. The catalytic center is such enzymes often involves metal hydroxides. | |||
| ==References== | ==References== | ||
Latest revision as of 06:12, 13 January 2025
Transition metal hydroxide complexes are coordination complexes containing one or more hydroxide (OH) ligands. The inventory is very large.
Hydroxide as a ligand
Hydroxide is classified as an X ligand in the Covalent bond classification method. In the usual electron counting method, it is a one-electron ligand when terminal and a three-electron ligand when doubly bridging.
From the electric structure perspective, hydroxide is a strong pi-donor ligand, akin to fluoride. One consequence is that few polyhydroxide complexes are low spin. Another consequence is that electron-precise hydroxide complexes tend to be rather nucleophilic.
Representative complexes
Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes.
Homoleptic complexes
Only a few homoleptic hydroxide complexes are known. These include the d species [Pt(OH)6] and the d complexes [Ti(OH)6] and [Zr2(OH)8(mu−OH)2].
Mixed ligand complexes
Many complexes are known where hydroxide shares the coordination sphere with other ligands. One pair of such complexes are {2(mu-OH)3} and its derivative {2(mu-OH)2}.
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Reactions
Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives aquo complexes:
- LnM−OH + H ⇌ LnM−OH+2 where Ln is the ligand complement on the metal M
Thus, aquo ligand is a weak acid, of comparable strength to acetic acid (pKa of about 4.8).
In principle but not very commonly, metal hydroxides undergo deprotonation, yielding oxo complexes:
- LnM−OH ⇌ LnM=O +H
Characteristically, hydroxide ligands are compact and basic. They tend to function as bridging ligands. One manifestation of this property is the preponderance of di-and polymetallic hydroxide complexes. A practical consequence of this feature is the tendency of metal aquo complexes to form precipitates of meta hydroxides.
Bioinorganic chemistry
Active site of hemerythrin before and after oxygenation.
Hemerythrins, proteins responsible for oxygen (O2) transport in some animals have an diiron hydroxide active site. The hydroxide ligand engages the bound O2 through hydrogen bonding.
The nucleophilicity of hydroxo ligands is relevant to the role of some M-OH centers in enzymology. For example, in carbonic anhydrase, a zinc hydroxide binds carbon dioxide:
- LnM−OH + CO2 ⇌ LnMO−O−CO2H
The oxygen evolving complex (OEC) consists of a Mn-Ca-O-OH cluster that is responsible for the biosynthesis of O2. It is proposed that the O-O bond forming step involves a hydroxide ligand.
Metalloproteinases catalyze the hydrolysis peptide bond. The catalytic center is such enzymes often involves metal hydroxides.
References
- ^ Nelson, David J.; Nolan, Steven P. (2017). "Hydroxide complexes of the late transition metals: Organometallic chemistry and catalysis" (PDF). Coordination Chemistry Reviews. 353: 278–294. doi:10.1016/j.ccr.2017.10.012.
- Fulton, J. Robin; Holland, Andrew W.; Fox, Daniel J.; Bergman, Robert G. (2002). "Formation, Reactivity, and Properties of Nondative Late Transition Metal−Oxygen and −Nitrogen Bonds". Accounts of Chemical Research. 35 (1): 44–56. doi:10.1021/ar000132x. PMC 1473979. PMID 11790088.
- Bandel, G.; Platte, C.; Trömel, M. (1982). "Ammonium hexahydroxoplatinat(IV) und Strukturverfeinerung für Kalium-hexahydroxoplatinat(IV)". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 38 (5): 1544–1546. Bibcode:1982AcCrB..38.1544B. doi:10.1107/S0567740882006311.
- Lin, Hechun; De Oliveira, Peter W.; Huch, Volker; Veith, Michael (2010). "Hydroxometalates from Anion Exchange Reactions of [BF4] based Ionic Liquids: Formation of [M(OH)6)] (M = Ti, Zr) and [Zr(OH)5]". Chemistry of Materials. 22 (24): 6518–6523. doi:10.1021/cm101490w.
- Wieghardt, K.; Siebert, H. (1985). μ-Carboxylatodi-μ-Hydroxo-Bis[Triamminecobalt(III)] Complexes. Inorganic Syntheses. Vol. 23. pp. 107–116. doi:10.1002/9780470132548.ch21. ISBN 978-0-471-81873-1.
- Lincoln, S. F.; Richens, D. T.; Sykes, A. G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. Vol. 1. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN 9780080437484.
- Parkin, Gerard (2004). "Synthetic Analogues Relevant to the Structure and Function of Zinc Enzymes". Chemical Reviews. 104 (2): 699–768. doi:10.1021/cr0206263. PMID 14871139.
| Coordination complexes | |
|---|---|
| H donors: | |
| B donors: | |
| C donors: | |
| Si donors: | |
| N donors: | |
| P donors: | |
| O donors: | |
| S donors: | |
| Halide donors: | |