Editing Pyridine (section)

===Lewis basicity and coordination compounds===
Pyridine is a [[Lewis base]], donating its pair of electrons to a Lewis acid. Its Lewis base properties are discussed in the [[ECW model]]. Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by [[ECW model|C-B plots]].<ref>Laurence, C. and Gal, J-F. (2010) ''Lewis Basicity and Affinity Scales, Data and Measurement''. Wiley. pp. 50–51. {{ISBN|978-0-470-74957-9}}</ref><ref>{{cite journal|author1=Cramer, R. E. |author2=Bopp, T. T. |year=1977|title= Graphical display of the enthalpies of adduct formation for Lewis acids and bases |journal= Journal of Chemical Education |volume=54|pages=612–613|doi= 10.1021/ed054p612}} The plots shown in this paper used older parameters. Improved E&C parameters are listed in [[ECW model]].</ref> One example is the [[sulfur trioxide pyridine complex]] (melting point 175&nbsp;°C), which is a [[sulfation]] agent used to convert alcohols to [[sulfate ester]]s. Pyridine-[[borane]] ({{chem2|C5H5NBH3}}, melting point 10–11&nbsp;°C) is a mild reducing agent.

[[File:Crabtree.svg|class=skin-invert-image|thumb|structure of the [[Crabtree's catalyst]]]]

[[Transition metal pyridine complexes]] are numerous.<ref name =nakamoto>{{cite book|last=Nakamoto|first=K.|title=Infrared and Raman spectra of Inorganic and Coordination compounds|edition=5th|series=Part A|year=1997|publisher=Wiley|isbn=0-471-16394-5}}</ref><ref>{{cite book|last=Nakamoto|first=K.|title=Infrared and Raman spectra of Inorganic and Coordination compounds|edition=5th|series=Part B|isbn=0-471-16392-9|page=24|date=31 July 1997}}</ref> 
Typical octahedral complexes have the stoichiometry {{chem2|MCl2(py)4}} and {{chem2|MCl3(py)3}}. Octahedral homoleptic complexes of the type {{chem2|M(py)6(+)}} are rare or tend to dissociate pyridine. Numerous square planar complexes are known, such as [[Crabtree's catalyst]].<ref>{{cite journal |last1=Crabtree |first1=Robert |title=Iridium compounds in catalysis |journal=Accounts of Chemical Research |volume=12 |pages=331–337 |year=1979 |doi=10.1021/ar50141a005|issue=9}}</ref> The pyridine ligand replaced during the reaction is restored after its completion.

The ''η''<sup>6</sup> coordination mode, as occurs in ''η''<sup>6</sup> benzene complexes, is observed only in [[Steric effects|sterically encumbered]] derivatives that block the nitrogen center.<ref name="Elschenbroich 2008 524–525">{{cite book|last=Elschenbroich |first=C. |title=Organometallchemie |edition=6th |pages=524–525 |publisher=Vieweg & Teubner |date=2008 |isbn=978-3-8351-0167-8}}</ref>