I'm currently writing up a report on Hydrocyanation and I've been reading up on parameters people regularly use to justify their observations in regard to the performance of certain catalyst systems without too much regard as to how they are defined.
That is how I came upon Chadwick Tolman's Electronic Parameter $\chi$, which I was told is defined as follows:
The electron donating or withdrawing effect of phosphorous ligands can be measures using IR-Spectroscopy by comparing the frequencies of the A1 C-O vibrational mode ($\nu(\ce{CO})$) of a complex $\ce{[LNi(CO)3]}$ and the reference complex $\ce{[Ni(CO)3P(tert-Bu)3]}$.
Ergo:
$\chi$ = $\nu$(CO,L) $-$ $\nu$($\ce{CO}$, $\ce{P(tert-Bu)3}$)
If $\chi > 0$, Ligand causes net withdrawal of electron density towards the metal center (in this case, Nickel) compared to the ligand $\ce{P(tert-Bu)3}$.
If $\chi < 0$, Ligand causes net donation of electron density towards the metal center.
The reason that we use $\ce{[Ni(CO)3P(tert-Bu)3]}$ as a reference and not $\ce{[Ni(CO)4]}$ is because we are interested in comparing it to other phosphorous ligands and not carbon monoxide, which isn't a very interesting ligand in homogenous catalysis.
So here are the two questions:
Is my formula correct? Because I cannot find a short formula that defines $\chi$ in a single paper.
Is the reason for $\ce{P(tert-Bu)3}$ as a reference correct?
