What is Tolman parameters?

The Tolman cone angle of a phosphine is the apex angle of the cone, centred on the metal at a standard distance, that just encloses the van der Waals surfaces of its substituents: a measure of its size. The Tolman electronic parameter is the wavenumber of the symmetric CO stretch of $\ce{Ni(CO)3L}$: the more strongly the phosphine L donates, the lower it is.