Site-selective coordination behaviour of the Py2P--anion: the N-C-P-allylic system as sigma- and pi-donor in [(PMDETA)Cs{(mu-PPy)Py}](2) and as a mu(2)-sigma-phosphorus-donor in [{Cp(CO)(2)Fe}(2){(mu-P)Py-2}][BMe4]

The coordination flexibility of the ambidentate Py2P--anion (Py = 2-pyridyl) spans the wide range from sigma -all-nitrogen chelation to hard organometallic moieties, sigma -phosphorus mu (2)-bridging in dinuclear iron complexes to pi -N,C,P heteroallyl coordination to the soft caesium.