the reactions of singlet silylenes, h/sub 2/si and f/sub 2/si, with fluoroborane, fbh/sub 2/, have been studied by ab initio molecular orbital methods using 3-21g and 6-31g*(5d) basis sets. the effects of electron correlation at selected geometries were determined by moller-plesset perturbation theory. the reactions of h/sub 2/si with fbh/sub 2/, leading to fh/sub 2/si-bh/sub 2/ by f atom transfer and to h/sub 3/si-bhf by h atom transfer, proceed without the formation of an adduct or transition state. the reactions of f/sub 2/si with fbh/sub 2/, leading to f/sub 3/si-bh/sub 2/ by f atom transfer and to hf/sub 2/si-bh/sub 2/ by h atom transfer, proceed with the formation of adducts (in the former case as a novel bridged structure) and transition states. in certain reactions f/sub 2/si behaves as a lewis acid and in others as a lewis base. the f atom transfer reaction has a 3.1 kcal/mol reaction barrier at the mp4sdq/6-31g* level and the h atom transfer reaction is found to have a 7.6 kcal/mol barrier at the same level. the structures of the fluorinated silylborane products as well as the adducts and transition states are given. the structure of perfluorosilylborane is presented and shown to be in excellent agreement with the structure determined by microwave spectroscopy