# Why are phosphines stronger-field ligands than water?

Mar 31, 2017

Because phosphines tend to be $\sigma$ donors and $\pi$ acceptors (at the same time). Water, on the other hand, is primarily a $\sigma$ donor, so it is a (MUCH) weaker-field ligand on the spectrochemical series than phosphines are, and cannot displace phosphines that are attached to a transition metal.

(Two other similar ligands in behavior are $\text{CO}$ and ${\text{CN}}^{-}$.)

A sigma ($\sigma$) donor is a molecule whose highest-occupied molecular orbital (HOMO) is a $\sigma$-type orbital, formed from two orbitals totally symmetric about the internuclear axis. Examples are overlaps of $n {p}_{z}$ atomic orbitals, $\left(n - 1\right) {d}_{{z}^{2}}$ atomic orbitals, etc.

$\sigma$ donors tend to come in and donate electron density into a ${\sigma}^{\text{*}}$ orbital.

A pi ($\pi$) acceptor is a molecule whose lowest-unoccupied molecular orbital (LUMO) is a ${\pi}^{\text{*}}$ type orbital, formed from two orbitals that overlapped sidelong. Examples are overlaps of $n {p}_{y}$ atomic orbitals along the $x$ axis, $\left(n - 1\right) {d}_{x z}$ atomic orbitals along the $x$ axis, etc.

$\pi$ acceptors tend to help stabilize the compound by accepting electron density into their ${\pi}^{\text{*}}$ LUMO.

Phosphines, however, are both of those at the same time. For example, triphenylphosphine, $P {h}_{3} P -$, have this capability. Consider $R = P h$ for the following:

$\boldsymbol{\sigma}$ donation:

$\boldsymbol{\pi}$ acceptance (i.e. backbonding):

The phenyl groups would help the $\pi$ acceptor ability of the phosphorus, since they can redistribute the delocalized electron density better than, say, if $R$ was alkyl, thereby stabilizing the bond order of the $P - C$ bond (thus counteracting the fact that electron density in the antibonding orbital increased, which would have weakened bond strength).

Water does not have the ability to $\pi$ backbond, so a phosphine's bond with a transition metal is more stabilized, and it is more resistant to incoming water ligands in a displacement reaction.