(Trimethylsilyl)phosphine (Me3SiPH2) undergoes radical P-H bond addition to vinylphosphines and -silanes to form new di-phospha- and 4-silaphosphorinanes [vinyl reagent] : [PhP(CH=CH2)(2)], PhP(C2H4)(2)PSiMe3 diastereomers (9A/9B); [Et2NP(CH=CH2)(2)], Et2NP(C2H4)(2)PSiMe3 (11); [Me2Si(CH=CH2)(2)], Me2Si(C2H4)(2)PSiMe3 (14); [Si(CH=CH2)(4)], (CH=CH2)(2)Si(C2H4)(2)PSiMe3 (16) and [Me3SiP(C2H4)(2)](2)Si (17) Reactions are accompanied by formation of only small quantities of the Markovnikov addition product phospholanes. Methanolysis of the new silylphosphines yields PhP(C2H4)(2)PH diastereomers (10A/1OB), Me2Si(C2H4)PH (15), (CH=CH2)(2)Si(C2H4)(2)-PH (18), and [HP(C2H4)(2)](2)Si(19). Stepwise methanolysis of 11 yields the phosphorinanes Et2NP(C2H4)2PH (12) and MeOP(C2H4)(2)PH (13). Oxidation of 15 and 14 with O-2 or O-2/H2O, respectively, yields the phosphine oxide Me2Si(C2H4)(2)P(O)H (20) and the phosphinic acid Me2Si(C2H4)(2)P(O)OH (21). New compounds were characterized by spectral ((31)p, H-1,,and C-13 NMR, IR, and MS) data. 21 was further characterized by a single-crystal X-ray analysis : monoclinic, P2(1)/c, a = 10.416(2) Angstrom, b = 6.817(1) Angstrom, c = 14.237(3) Angstrom, beta = 106.32(2)degrees, Z = 4, V = 970.3(3) Angstrom(3). The ring of 21 adopts a chair conformation with the P=O bond in an equatorial position. From spectral data, tentative isomeric and conformational structural assignments are made for the new phosphorinanes in solution.