Transition metal pyridine complexes

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.

Structure of [Ru(NH₃)₅py]²⁺, illustrating the steric avoidance of the 2,6-protons and the cis ligands.^[1]

Bonding

With a pK_a 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] Low-valent metal complexes of pyridines are known, e.g. Ir^I(diene)(pyridine)Cl. 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.

Pyridine is classified as L ligand in the Covalent bond classification method. In the usual electron counting method, it is a two-electron ligand. With respect to HSAB theory, it is intermediate softness, reflecting its small but significant properties as a pi-acceptor ligand.

Classification of metal-pyridine complexes

Many metal pyridine complexes are known. These complexes can be classified according to their geometry, i.e. octahedral, tetrahedral, linear, etc.

Octahedral complexes

trans-[MCl₂(pyridine)₄]ⁿ⁺ is a common type of transition metal pyridine complex.

Chloro(pyridine)cobaloxime.

Crabtree's catalyst.

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.

MCl₂(pyridine)₄ 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

Collins reagent, the complex CrO₃(pyridine)₂, is a reagent in organic chemistry.^[20]

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]

MCl₂py₂ + 2 py → MCl₂py₄

Two- and three-coordinate complexes

Many examples exist for [Au(py)₂]⁺.^[27] [Ag(py)₃]⁺ and [Cu(py)₂]⁺ are also precedented.^[30][27]

Pi-complexes

The η⁶ coordination mode, as occurs in η⁶ benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center.^[31]

Comparison with related ligands

Picolines

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]

2,2'-bipy

Main article: Transition metal complexes of 2,2'-bipyridine

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

Imidazoles comprise another major series of N-heterocyclic ligands. Unlike pyridines, imidazole derivatives are common ligands in nature.

Applications and occurrence

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]