Transition metal pyridine complexes
Transition metal pyridine complexes encompas many coordination complexess 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 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 MCl24]2+ reveal an anticorrelation with d-electron count. 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 and pyridine adduct of borane, BH3py.Octahedral complexes
Owing to the relatively wide C-N-C angle, the 2,6-hydrogen atoms interfere with the adjacent ligands. Consequently, relatively few octahedral homoleptic pyridine complexes are known. These include 2+. and. Far more common are complexes of the type n+, which are invariably trans. Other common classes of complexes are the tris trihalides .| formula | CAS RN | density from X-ray | - |
| TiCl24 | 131618-68-3 | 1.383 | - |
| VCl24 | 15225-42-0 | 1.346 | - |
| CrCl24.acetone | 51266-53-6 | 1.383 | - |
| MnCl24 | 14638-48-3 | 1.383 | - |
| FeCl24 | 15138-92-8 | 1.383 | - |
| CoCl24 | 13985-87-0 | 1.383 | - |
| NiCl24 | 14076-99-4 | 1.383 | - |
| Dichlorotetrakisrhodium chloride|+ | 14077-30-6 | - | |
| RuCl24]+ | isolated as the salt with − | - | |
| NbCl24 | 168701-43-7 | 1.528 |
Four-coordinate complexes
Four-coordinate complexes include tetrahedral and square planar derivatives. Examples of homoleptic tetrahedral complexes include n+ for Mn+ = Cu+, M = Ni2+, Ag+. Examples of homoleptic square planar complexes include the d8 cations n+ for Mn+ = Pd2+, Pt2+, Au3+.Two-coordinate complexes
Examples include the homoleptic complexes of d10 metal centers. Examples include +, which form colorless, diamagnetic salts.Applications and occurrence
Transition metal pyridine complexes have few practical applications, but they are widely used precursors to varioous homogeneous catalysts. Crabtree's catalyst, a popular catalyst for hydrogenations, is a pyridine complex.Unlike imidazoles, pyridine and its derivatives are uncommon ligands in nature.