The reaction of [Bu2Sn (OPr)] 2O with RPO3H2 (R= Me, Ph), in Sn/P molar ratios of 1: 1 or 1: 2, yields di-n-butyltin phosphonates or hydrogenophosphonates. The molecular structure detemined by X-ray diffraction for [Bu2Sn (HO3PMe) 2] 2, as well as multinuclear solid state and solution NMR studies, including 31P-119Sn HMQC experiments, systematically reveals a trigonal bipyramidal configuration at tin, with a very strong stereochemical fluxionality for ...