Reaction of Z-ClBEtCEt:CRSiMe2Cl (R = Me, CMe:CH2) with PhPLi2 gave 60-80% dimers of title compds. (I; same R), which when treated with 4-picoline or Me3P gave monomeric adducts. Treating I (R = Me) (II) with Ph3P:CH2 and then heating gave 70% bis(tetrahydrophosphasilaborin) III. Reactions of II with electrophiles are discussed. The structures of II and III were detd. by 1H, 13C, 11B, and 31P NMR and by x-ray crystallog.