We have studied the coordination of substituted pyridines, and phosphorus-
and sulfur-containing ligands to an SCS Pd-II pincer system. These ligands
coordinate to Pd-II (trans to the cyclopalladated aryl group) by quantitati
ve substitution of the labile acetonitrile ligand in complex 1. Competition
experiments showed that both electronic and steric effects influence the s
trength of coordination to the Pd-II pincer of the substituted pyridines. A
quantitative analysis of the substituent effect was achieved by a Hammett
correlation. Phosphorus-containing Ligands also coordinate to this SCS Pd-I
I motif, as evidenced by NMR spectroscopy and single-crystal X-ray diffract
ion studies. They are much stronger Ligands than the pyridines. The coordin
ation strength of the thioureas falls in between those of the pyridines and
phosphanes/phosphites. Our results lead therefore to the following order o
f ligand strength towards Pd-II in SCS Pd-II pincers: PR3 > P(OR)(3) > N,N'
-disubstituted thiourea > (substituted) pyridines > MeCN.