effect of ligands on metal NMR shift

[king87]

transition metal complexes, effect of ligands on metal NMR shift... complexes of formula M(amine)2X2, they write, that the difference between cis and trans isomers chemical shift, for dipiridine isomers (amine=piridine) is greater for diiodo than for dichloro complexes... and say if ligands amine and X are far from each other in the trans influence series, this difference chemical shift cis-trans is bigger... but why? wow it influence metal chemical shift? trans influenceof iodine is much larger than chlorine, so the Pt-N bond trans to iodine will be longer than when its trans to chlorine. but how it affects shielding of nmr signal? i would say its just withdrawal of electrons, why order in trans series affects that?