The driving force for the condensation reaction may be viewed as a sum of two contributions (positive or negative) -the exchange of the reactive group M-X for the silicate moiety M-O-Si on the metal (different for each precursor) and the exchange of the silicate moiety Sn-O-Si for Sn-X on the trimethyltin group (intrinsic driving force, common to all precursors). Reactions 26A and 26B model the latter process for precursors with Cl -as a leaving group to assess how much of the driving force (and more importantly with what sign) comes intrinsically from the chemistry of the trimethyltin group.