# 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.

## 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 [MCl$_2$(py)$_4$]$^{2+}$ reveal an anticorrelation with d-electron count. 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 SO$_3$(py) and pyridine adduct of borane, BH$_3$py.
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

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)$_6$]Fe$_4$(CO)$_{13}$ and [Ru(py)$_6$](BF$_4$)$_2$. Some compounds with the stoichiometry M(py)$_6$(ClO$_4$)$_2$ have been reformulated as [M(py)$_4$(ClO$_4$)$_2$]$^.$(py)$_2$
A common family of pyridine complexes are of the type [MCl$_2$(py)$_4$]$^{n+}$. The chloride ligands are mutually trans in these complexes.
MCl$_2$(pyridine)$_4$ complexes :
| formula | CAS RN | key properties | Preparation |
| TiCl$_2$(pyridine)$_4$ | 131618-68-3 | blue, triplet d$_{Ti-N}$=2.27 Å, d$_{Ti-Cl}$ = 2.50 Å (thf solvate) | TiCl$_3$(thf)$_3$ + KC$_8$ + py |
| VCl$_2$(pyridine)$_4$ | 15225-42-0 | purple | VCl$_3$ + Zn + py |
| CrCl$_2$(pyridine)$_4$ | 51266-53-6 | green d$_{Cr-Cl}$ = 2.80 Å d$_{Co-Cl}$ = 2.16 Å | CrCl$_2$ + py |
| MnCl$_2$(pyridine)$_4$ | 14638-48-3 | 1.383 |
| FeCl$_2$(pyridine)$_4$ | 15138-92-8 | yellow d$_{Fe-Cl}$ = 2.43 Å | FeCl$_2$ + py |
| CoCl$_2$(pyridine)$_4$ | 13985-87-0 | blue d$_{Co-Cl}$ = 2.44 Å | CoCl$_2$ + py |
| [CoCl$_2$(pyridine)$_4$]Cl | 27883-34-7 | green (hexahydrate) d$_{Co-Cl}$ = 2.25 Å, d$_{Co-N}$ = 1.98 Å as [CoCl$_3$(py)]$^−$ salt | CoCl$_2$(pyridine)$_4$ + Cl$_2$ |
| NiCl$_2$(pyridine)$_4$ | 14076-99-4 | blue d$_{Ni-Cl}$ = 2.44 Å | NiCl$_2$ + py |
| NbCl$_2$(pyridine)$_4$ | 168701-43-7 | d$_{Nb-N}$ = 2.22 Å, d$_{Nb-Cl}$ = 2.51 Å | NbCl$_4$(thf)$_2$ + KC$_8$ + py |
| [MoCl$_2$py)$_4$]Br$_3$ | | Br$_3$$^−$ salt | yellow d$_{Mo-Cl}$= 2.41 Å, d$_{Mo-N}$=2.20 Å |
| TcCl$_2$py)$_4$ | 172140-87-3 | purple d$_{Tc-Cl}$ = 2.41 Å, d$_{Tc-N}$ = 2.10 Å | TcCl$_4$py$_2$ + Zn + py |
| RuCl$_2$(pyridine)$_4$ | 16997-43-6 | red-orange d$_{Ru-N}$=2.08 Å, d$_{Ru-Cl}$=2.40 Å | RuCl$_3$(H$_2$O)$_x$ + py |
| [RhCl$_2$(pyridine)$_4$]$^+$ | 14077-30-6 (Cl$^−$ salt) | yellow | RhCl$_3$(H$_2$O)$_3$ + py + cat. reductant |
| OsCl$_2$(pyridine)$_4$ | 137822-02-7 | brown d$_{Os-Cl}$ = 2.40 Å, d$_{Os-N}$= 2.068 Å | K$_3$OsCl$_6$ + py + (CH$_2$OH)$_2$/140 °C |
| [IrCl$_2$(pyridine)$_4$]$^+$ | | yellow 1.35 Å (chloride$^.$hexahydrate) | |
The tris(pyridine) trihalides, i.e., [MCl$_3$(py)$_3$] (M = Ti, Cr, Rh Ir), are another large class of M-Cl-py complexes.

### Four-coordinate complexes

Four-coordinate complexes include tetrahedral and square planar derivatives. Examples of homoleptic tetrahedral complexes include [M(py)$_4$]$^{n+}$ for M$^{n+}$ = Cu$^+$, M = Ni$^{2+}$, Ag$^+$, and Ag$^{2+}$. Examples of homoleptic square planar complexes include the d$^8$ cations [M(py)$_4$]$^{n+}$ for M$^{n+}$ = Pd$^{2+}$, Pt$^{2+}$, Au$^{3+}$.
Ni(ClO$_4$)$_2$(3-picoline)$_2$ can be isolated in two isomers, yellow, diamagnetic square planar or blue, paramagnetic tetrahedral.
Mn(II) and Co(II) form both tetrahedral MCl$_2$py$_2$ and octahedral MCl$_2$py$_4$ complexes, depending on conditions:
    MCl$_2$py$_2$ + 2 py → MCl$_2$py$_4$

### Two- and three-coordinate complexes

Many examples exist for [Au(py)$_2$]$^+$. [Ag(py)$_3$]$^+$ and [Cu(py)$_2$]$^+$ are also precedented.

### Pi-complexes

The η$^6$ coordination mode, as occurs in η$^6$ benzene complexes, is observed only in sterically encumbered derivatives that block the nitrogen center.

## 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.

### 2,2'-bipy

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 [MCl$_2$(bipy)$_2$]$^{n+}$ tend to be cis, as exemplified by RuCl$_2$(bipy)$_2$. In contrast, the complexes [MCl$_2$(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 CoCl$_2$(py)4 has proven very useful in organocobalt chemistry and NiCl$_2$(py)$_4$ useful in organonickel chemistry.