1:1 and 1:2 adducts of diphenyltin dichloride with triphenylphosphine oxide can be isolated in the solid state but the preparative conditions necessary to isolate an adduct of specific stoichiometry are difficult to define; none of acid: base ratio, choice of solvent or reaction temperature are significant factors. Adventitious crystal seeding probably has a dominant role. Both the 1:1 and 1:2 adducts of SnPh2Cl2, but only the 1:1 adduct of SnPh2Br2, were isolated. 1:1 and 1:2 adducts of both SnMe2Cl2 and (SnBu2Cl2)-Cl-n were readily isolated whereas only 1:2 adducts were obtained with SnPh2(NCS)(2) and SnBu2n(NCS)(2). SbPh2Cl3 readily forms a 1:1 adduct with triphenylphosphine oxide and the same adduct is obtained from the reaction of the phosphine oxide with SbPhCl2. Crystallographic data are reported for SnPh2Cl2. OPPh3, SnPh2Cl2. 2OPPh(3), SnPh2Br2. OPPh3 and SbPh3Cl3. OPPh3. Both SnPh2Cl2. OPPh3 and SnPh2Br2. OPPh3 have tin in trigonal bipyramidal environments. Sn-119 Mossbauer quadrupole splitting data are consistent with the same trigonal bipyramidal geometry for tin in SnMe2Cl2. OPPh3 whereas the data for the butyl analogue suggest the presence of six-coordinated tin as a result of intermolecular Sn-CI interactions. A notable feature of all of the adducts subjected to crystallographic studies is the presence of intramolecular re-interactions involving phenyl groups of the phosphine oxide and phenyl groups bonded to the metal. (C) 2000 Elsevier Science B.V. All rights reserved.