Abstract: Nucleosidyl phosphite-borane compounds of the formula: ##STR1## wherein: Nucleoside is a natural or synthetic nucleoside connected to the phosphorus atom via a hydroxyl oxygen; each X is independently selected from O and BHR.sub.3 R.sub.4 ;R.sub.1 is selected from H, alkyl, aryl, alkyaryl, monovalent metal ions, and ammonium cation; R.sub.2 is selected from OR.sub.1 and N(R.sub.5).sub.2, wherein R.sub.5 is independently selected from H, C.sub.1 -C.sub.10 linear or branched alkyl or aryl; R.sub.3 is selected from H, CN, COOH, carboxyl salts, COOR.sub.6 and CONHR.sub.6, wherein R.sub.6 is selected from H, C.sub.1 -C.sub.10 alkyl, alkylaryl and aryl; R.sub.4 is selected from H and C.sub.1 -C.sub.10 alky; and n is 1 to 2.