Editing Pi backbonding (section)

==Metal carbonyls, nitrosyls, and isocyanides==
[[File:CO-M σ bonding.png|thumb|σ bonding from electrons in CO's HOMO to metal center d-orbital.]]
[[File:CO-M Pi Backbond.png|thumb|π backbonding from electrons in metal center d-orbital to CO's LUMO.]]
The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogs). This electron-transfer strengthens the metal–C bond and weakens the C–O bond. The strengthening of the M–CO bond is reflected in increases of the vibrational frequencies for the M–C bond (often outside of the range for the usual IR spectrophotometers). Furthermore, the M–CO bond length is shortened. The weakening of the C–O bond is indicated by a decrease in the wavenumber of the ''ν''<sub>CO</sub> band(s) from that for free CO (2143&nbsp;cm<sup>−1</sup>), for example to 2060&nbsp;cm<sup>−1</sup> in Ni(CO)<sub>4</sub> and 1981&nbsp;cm<sup>−1</sup> in Cr(CO)<sub>6</sub>, and 1790&nbsp;cm<sup>−1</sup> in the anion [Fe(CO)<sub>4</sub>]<sup>2−</sup>.<ref>{{cite book|last1=Housecroft |first1=C. E. |last2=Sharpe |first2=A. G. |title=Inorganic Chemistry |edition=2nd |publisher=Pearson Prentice-Hall |date=2005 |page=702 |isbn=978-0-130-39913-7}}</ref> For this reason, [[IR spectroscopy]] is an important diagnostic technique in [[metal carbonyl|metal–carbonyl chemistry]]. The article [[infrared spectroscopy of metal carbonyls]] discusses this in detail.

Many ligands other than CO are strong "backbonders". Nitric oxide is an even stronger π-acceptor than CO and ν<sub>NO</sub> is a diagnostic tool in [[Metal nitrosyl complex|metal–nitrosyl chemistry]]. [[Isocyanide]]s, RNC, are another class of ligands that are capable of π-backbonding. In contrast with CO, the σ-donor lone pair on the C atom of isocyanides is antibonding in nature and upon complexation the CN bond is strengthened and the ν<sub>CN</sub> increased. At the same time, π-backbonding lowers the ''ν''<sub>CN</sub>. Depending on the balance of σ-bonding versus π-backbonding, the ν<sub>CN</sub> can either be raised (for example, upon complexation with weak π-donor metals, such as Pt(II)) or lowered (for example, upon complexation with strong π-donor metals, such as Ni(0)).<ref>{{cite book|first=Robert H. |last=Crabtree |title=The Organometallic Chemistry of the Transition Metals |edition=6th |publisher=Wiley |date=2014 |pages=105–106 |isbn=978-1-11813807-6}}</ref> For the isocyanides, an additional parameter is the MC=N–C angle, which deviates from 180° in highly electron-rich systems. Other ligands have weak π-backbonding abilities, which creates a labilization effect of CO, which is described by the [[cis effect|''cis'' effect]].