The reactions of PhBCl2with Li[CE(NtBu)(nBu)] or Li[CS(NtBu)(NHtBu)] (1:1 molar ratio) in toluene at 23°C produced the heterocycles Ph(Cl)B(&mgr;-NtBu)(&mgr;-E)C(nBu) (1a, E = NtBu;1b, E = O;1c, E = S) or Ph(Cl)B(&mgr;-NtBu)(&mgr;-S)C(NHtBu) (2), which were characterized by1H,11B and13C NMR and by mass spectra. X-ray structural determinations revealed that1a,1cand2contain four-membered rings. In2the thioamidate ligand adopts anN,Sbonding mode. Crystal data:1a, monoclinic, space groupP21,a= 8.816(3),b= 11.311(2),c= 10.168(3) Å, &bgr; = 98.86(3)°,V= 1001.7(5) Å3,Z= 2,R= 0.042, andRw= 0.020;1c, monoclinic, space groupP21/n,a= 7.617(2),b= 11.200(1),c= 19.568(2), &bgr; = 90.74(2)°,V= 1669.1(5) Å3,Z= 4,R= 0.046, andRw= 0.059;2, monoclinic, space groupP21/a,a= 11.357(2),b= 12.289(2),c= 12.620(3) Å, &bgr; = 95.43(2)°,V= 1753.4(5) Å3,Z= 4,R= 0.043, andRw= 0.027.Key words: boron, amidinate, oxoamidate, thioamidate, X-ray st