![Structure of [Ru(NH3)5py] , illustrating the steric avoidance of the 2,6-protons and the cis ligands. Ru(NH3)5py(OTf)2Xray.png](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Ru%28NH3%295py%28OTf%292Xray.png/132px-Ru%28NH3%295py%28OTf%292Xray.png)
Transition metal pyridine complexes encompass many coordination complexes that contain pyridine as a ligand. Most examples are mixed-ligand complexes. Many variants of pyridine are also known to coordinate to metal ions, such as the methylpyridines, quinolines, and more complex rings.
With a pKa of 5.25 for its conjugate acid, pyridine is about 15x less basic than imidazole. Pyridine is a weak pi-acceptor ligand. Trends in the M-N distances for complexes of the type [MCl2(py)4]2+ reveal an anticorrelation with d-electron count. [2] Few low-valent metal complexes of pyridines are known. The role of pyridine as a Lewis base extends also to main group chemistry. Examples include sulfur trioxide pyridine complex SO3(py) and pyridine adduct of borane, BH3py.
Many metal pyridine complexes are known. These complexes can be classified according to their geometry, i.e. octahedral, tetrahedral, linear, etc.
![trans-[MCl2(pyridine)4] is a common type of transition metal pyridine complex. MCl2py4generic.png](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/MCl2py4generic.png/220px-MCl2py4generic.png)
Owing to the relatively wide C-N-C angle, the 2,6-hydrogen atoms interfere with the formation of [M(py)6]z complexes. A few octahedral homoleptic pyridine complexes are known. These complex cations are found in the salts [Ru(py)6]Fe4(CO)13 and [Ru(py)6](BF4)2. [3] [4] Some compounds with the stoichiometry M(py)6(ClO4)2 have been reformulated as [M(py)4(ClO4)2].(py)2 [5]
A common family of pyridine complexes are of the type [MCl2(py)4]n+. The chloride ligands are mutually trans in these complexes.
| formula | CAS RN | key properties | Preparation | |
|---|---|---|---|---|
| TiCl2(pyridine)4 | 131618-68-3 | blue, triplet dTi-N=2.27 Å, dTi-Cl = 2.50 Å (thf solvate) [6] | TiCl3(thf)3 + KC8 + py [7] | |
| VCl2(pyridine)4 | 15225-42-0 | purple [8] | VCl3 + Zn + py [9] | |
| CrCl2(pyridine)4 | 51266-53-6 | green dCr-Cl = 2.80 Å dCo-Cl = 2.16 Å | CrCl2 + py [10] | |
| MnCl2(pyridine)4 | 14638-48-3 | 1.383 | ||
| FeCl2(pyridine)4 | 15138-92-8 | yellow dFe-Cl = 2.43 Å | FeCl2 + py [2] | |
| CoCl2(pyridine)4 | 13985-87-0 | blue dCo-Cl = 2.44 Å | CoCl2 + py [2] | |
| [CoCl2(pyridine)4]Cl | 27883-34-7 | green (hexahydrate) dCo-Cl = 2.25 Å, dCo-N = 1.98 Å [11] as [CoCl3(py)]− salt | CoCl2(pyridine)4 + Cl2 [12] | |
| NiCl2(pyridine)4 | 14076-99-4 | blue dNi-Cl = 2.44 Å | NiCl2 + py [2] | |
| NbCl2(pyridine)4 | 168701-43-7 | dNb-N = 2.22 Å, dNb-Cl = 2.51 Å | NbCl4(thf)2 + KC8 + py [6] | |
| [MoCl2py)4]Br3 | Br3− salt [13] | yellow dMo-Cl= 2.41 Å, dMo-N=2.20 Å | ||
| TcCl2py)4 | 172140-87-3 | purple dTc-Cl = 2.41 Å, dTc-N = 2.10 Å [14] | TcCl4py2 + Zn + py | |
| RuCl2(pyridine)4 | 16997-43-6 | red-orange dRu-N=2.08 Å, dRu-Cl=2.40 Å | RuCl3(H2O)x + py [15] | |
| [RhCl2(pyridine)4]+ | 14077-30-6 (Cl− salt) | yellow | RhCl3(H2O)3 + py + cat. reductant [16] | |
| OsCl2(pyridine)4 | 137822-02-7 | brown dOs-Cl = 2.40 Å, dOs-N= 2.068 Å | K3OsCl6 + py + (CH2OH)2/140 °C [17] | |
| [IrCl2(pyridine)4]+ | yellow 1.35 Å (chloride.hexahydrate) [18] |
The tris(pyridine) trihalides, i.e., [MCl3(py)3] (M = Ti, Cr, Rh [19] Ir), are another large class of M-Cl-py complexes.
Four-coordinate complexes include tetrahedral and square planar derivatives. Examples of homoleptic tetrahedral complexes include [M(py)4]n+ for Mn+ = Cu+, [21] M = Ni2+, [22] Ag+, [23] and Ag2+. [24] Examples of homoleptic square planar complexes include the d8 cations [M(py)4]n+ for Mn+ = Pd2+, [25] Pt2+, [26] Au3+. [27]
Ni(ClO4)2(3-picoline)2 can be isolated in two isomers, yellow, diamagnetic square planar or blue, paramagnetic tetrahedral. [28]
Mn(II) and Co(II) form both tetrahedral MCl2py2 and octahedral MCl2py4 complexes, depending on conditions: [29]
Many examples exist for [Au(py)2]+. [27] [Ag(py)3]+ and [Cu(py)2]+ are also precedented. [30] [27]
The η6 coordination mode, as occurs in η6 benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center. [31]
Many substituted pyridines function as ligands for transition metals. The monomethyl derivatives, the picolines (2-, 3-, and 4-picoline), are best studied. 2-Picolines are sterically impeded from coordination. [28]
Coupling of two pyridine rings at their 2-positions gives 2,2'-bipyridine, a widely studied bidentate ligand. A number of differences are apparent between pyridine and bipyridine complexes. Many [M(bipy)3]z complexes are known, whereas analogous [M(py)6]z complexes are rare and apparently labile. Bipyridine is a redox-noninnocent ligand, as illustrated by the existence of complexes such as [Cr(bipy)3]0. The pyridine analogues of such complexes are unknown. The dichloro complexes [MCl2(bipy)2]n+ tend to be cis, as exemplified by RuCl2(bipy)2. In contrast, the complexes [MCl2(py)4]n+ are always trans.
Imidazoles comprise another major series of N-heterocyclic ligands. Unlike pyridines, imidazole derivatives are common ligands in nature.
Crabtree's catalyst, a popular catalyst for hydrogenations, is a pyridine complex.
Although transition metal pyridine complexes have few practical applications, they are widely used synthetic precursors. Many are anhydrous, soluble in nonpolar solvents, and susceptible to alkylation by organolithium and Grignard reagents. Thus CoCl2(py)4 has proven very useful in organocobalt chemistry [32] [33] and NiCl2(py)4 useful in organonickel chemistry. [34]
In chemistry, water(s) of crystallization or water(s) of hydration are water molecules that are present inside crystals. Water is often incorporated in the formation of crystals from aqueous solutions. In some contexts, water of crystallization is the total mass of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex or a salt, which is not directly bonded to the metal cation.
Cobalt(II) chloride is an inorganic compound, a salt of cobalt and chlorine, with the formula CoCl
2. The compound forms several hydrates CoCl
2·nH
2O, for n = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed. The anhydrous form is a blue crystalline solid; the dihydrate is purple and the hexahydrate is pink. Commercial samples are usually the hexahydrate, which is one of the most commonly used cobalt salts in the lab.
Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.
1,10-Phenanthroline (phen) is a heterocyclic organic compound. It is a white solid that is soluble in organic solvents. The 1,10 refer to the location of the nitrogen atoms that replace CH's in the hydrocarbon called phenanthrene.
In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.
Tetrakis(acetonitrile)copper(I) hexafluorophosphate is a salt with the formula [Cu(CH3CN)4]PF6. It is a colourless solid that is used in the synthesis of other copper complexes. The cation [Cu(CH3CN)4]+ is a well-known example of a transition metal nitrile complex.
Bis(triphenylphosphine)palladium chloride is a coordination compound of palladium containing two triphenylphosphine and two chloride ligands. It is a yellow solid that is soluble in some organic solvents. It is used for palladium-catalyzed coupling reactions, e.g. the Sonogashira–Hagihara reaction. The complex is square planar. Many analogous complexes are known with different phosphine ligands.
Dichlorotris(triphenylphosphine)ruthenium(II) is a coordination complex of ruthenium. It is a chocolate brown solid that is soluble in organic solvents such as benzene. The compound is used as a precursor to other complexes including those used in homogeneous catalysis.
Transition metal nitrile complexes are coordination compounds containing nitrile ligands. Because nitriles are weakly basic, the nitrile ligands in these complexes are often labile.
cis-Dichlorobis(bipyridine)ruthenium(II) is the coordination complex with the formula RuCl2(bipy)2, where bipy is 2,2'-bipyridine. It is a dark green diamagnetic solid that is a precursor to many other complexes of ruthenium, mainly by substitution of the two chloride ligands. The compound has been crystallized as diverse hydrates.
Dichlorotetrakis(pyridine)iron(II) is the coordination complex with the formula FeCl2(pyridine)4. A yellow solid, it is a prominent example of a transition metal pyridine complex. It is used as an anhydrous precursor to other iron complexes and catalysts. According to X-ray crystallography, the chloride ligands are mutually trans. The complex has a high spin configuration. A monohydrate as well as several related complexes are known, e.g. CoCl2(pyridine)4 and NiCl2(pyridine)4. It is prepared by treating ferrous chloride with an excess of pyridine.
Dichlorotetrakis(pyridine)rhodium(III) chloride is the chloride salt of the coordination complex with the formula [RhCl2(pyridine)4]+. Various hydrates are known, but all are yellow solids. The tetrahydrate initially crystallizes from water. The tetrahydrate converts to the monohydrate upon vacuum drying at 100 °C.
Nitrate chlorides are mixed anion compounds that contain both nitrate (NO3−) and chloride (Cl−) ions. Various compounds are known, including amino acid salts, and also complexes from iron group, rare-earth, and actinide metals. Complexes are not usually identified as nitrate chlorides, and would be termed chlorido nitrato complexes.
In chemistry, a transition metal chloride complex is a coordination complex that consists of a transition metal coordinated to one or more chloride ligand. The class of complexes is extensive.
Transition metal thioether complexes comprise coordination complexes of thioether (R2S) ligands. The inventory is extensive.
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In chemistry, a transition metal ether complex is a coordination complex consisting of a transition metal bonded to one or more ether ligand. The inventory of complexes is extensive. Common ether ligands are diethyl ether and tetrahydrofuran. Common chelating ether ligands include the glymes, dimethoxyethane (dme) and diglyme, and the crown ethers. Being lipophilic, metal-ether complexes often exhibit solubility in organic solvents, a property of interest in synthetic chemistry. In contrast, the di-ether 1,4-dioxane is generally a bridging ligand.
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