[tvtokyo]

Hi, I am facing problem of understanding the 31P-NMR chemical shift of the Phosphorus in this complex:



I know that there are two P(a) which are equivalent and the other Phosphorus is then P(b).

The result of 31P NMR is as bellow:
P(a) is more deshielded than P(b) since the chemical shift of P(a) is 50+ ppm and P(b) is 40+ ppm. Can someone point me in the right direction - which theory can I use to explain why P(a) is more deshielded? Thanks

[Flatbutterfly]

The effective magnetic field at a spin½ nuclei is given by Beff  = (1- σ)Bo where Bo is the applied magnetic field and σ is the shielding parameter.  The σ term however contains several terms (e.g., σsolv, σ(ring current)) but the terms of most concern are σdia and σpara.  σdia concerns the spherical e⁻ density around the nucleus: an increase in the  e⁻ density results in increased shielding and the resonance is shifted upfield.  For H this is the most important term because H bonds via the spherically symmetric 1s AO.  For ALL other spin½ nuclei including P-31 the paramagnetic term σpara dominates (the term has nothing to do with unp e⁻); it concerns the asymmetric circulation of e⁻ density.  If your P-31 shifts are consistent with simple ± e⁻ donating arguments it is a coincidence!  The paramagnetic term contains a 1/ΔE term where ΔE is the HOMO-LUMO gap (would I kid you?) and p “imbalance” terms (for TM cmpds throw in d orbital imbalance as well).  The σpara has the opposite sign to σdia and so a large σpara causes the resonance to shift downfield (by huge amounts: think nothing of 200 ppm vs. ~10 ppm for σdia).
For your cmpds you may therefore get further by relating the shifts to the Ru 4d → π* HOMO-LUMO gap: the smaller the ΔE the further the shift downfield.  But to do a reasonable job you have to do MO calcs.
www.oci.uzh.ch/group.pages/zerbe/NMR.pdf 
This person (pages 54-55) gets it right!!