Lanthanide versus actinide reactivity in the formation of sterically crowded [(C(5)Me(5))(3)ML(n)] nitrile and isocyanide complexes.

The limits of steric crowding in organometallic metallocene complexes have been examined by studying the synthesis of [(C(5)Me(5))(3)ML(n)] complexes as a function of metal in which L=Me(3)CCN, Me(3)CNC, and Me(3)SiCN. The bis(tert-butyl nitrile) complexes [(C(5)Me(5))(3)Ln(NCCMe(3))(2)] (Ln=La, 1; Ce, 2; Pr, 3) can be isolated with the largest lanthanide metal ions, La(3+), Ce(3+), and Pr(3+). The Pr(3+) ion also forms an isolable mono-nitrile complex, [(C(5)Me(5))(3)Pr(NCCMe(3))] (4), whereas for Nd(3+) only the mono-adduct [(C(5)Me(5))(3)Nd(NCCMe(3))] (5) was observed. With smaller metal ions, Sm(3+) and Y(3+), insertion of Me(3)CCN into the M--C(C(5)Me(5)) bond was observed to form the cyclopentadiene-substituted ketimide complexes [(C(5)Me(5))(2)Ln{NC(C(5)Me(5))(CMe(3))}(NCCMe(3))] (Ln=Sm, 6; Y, 7). With tert-butyl isocyanide ligands, a bis-isocyanide product can be isolated with lanthanum, [(C(5)Me(5))(3)La(CNCMe(3))(2)] (8), and a mono-isocyanide product with neodymium, [(C(5)Me(5))(3)Nd(CNCMe(3))] (9). Silicon-carbon bond cleavage was observed in reactions between [(C(5)Me(5))(3)Ln] complexes and trimethylsilyl cyanide, Me(3)SiCN, to produce the trimeric cyanide complexes [{(C(5)Me(5))(2)Ln(mu-CN)(NCSiMe(3))}(3)] (Ln=La, 10; Pr, 11). With uranium, a mono-nitrile reaction product, [(C(5)Me(5))(3)U(NCCMe(3))] (12), which is analogous to 5, was obtained from the reaction between [(C(5)Me(5))(3)U] and Me(3)CCN, but [(C(5)Me(5))(3)U] reacts with Me(3)CNC through C--N bond cleavage to form a trimeric cyanide complex, [{(C(5)Me(5))(2)U(mu-CN)(CNCMe(3))}(3)] (13).