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 MC(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(μ‐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 CN bond cleavage to form a trimeric cyanide complex, [{(C 5 Me 5 ) 2 U(μ‐CN)(CNCMe 3 )} 3 ] ( 13 ).