Density Functional Theory and X-ray Analysis of the Structural Variability in η5,η5,η1-Tris(ring) Rare Earth/Actinide Tetramethylpyrrolyl Complexes, (C5Me5)2M(NC4Me4)

(C5Me5)2M­(NC4Me4) complexes (M = Y, Sm, Ce, U) were synthesized to act as structural models for the (η5-C5Me5)2M­(η1-C5Me5) intermediate postulated to give pseudoalkyl reactivity to sterically crowded (C5Me5)3M complexes. This synthesis was accomplished through reaction of the tetraphenylborate complexes, [(C5Me5)2M]­[(μ-Ph)2BPh2], with potassium tetramethylpyrrolyl, KNC4Me4. X-ray crystallographic studies on the resulting (C5Me5)2M­(NC4Me4) complexes showed that, although the two (C5Me5)− rings bind to the metal with η5 coordination and tetramethylpyrrolyl has a primary η1 coordination, the complexes are not symmetrical in the solid state, and disparate M–N–C­(ring) angles within a complex orient a (NC4Me4)− ring carbon and methyl carbon near the metal in a pseudo-η3 binding mode. Moreover, these (C5Me5)2M­(NC4Me4) complexes display unexpectedly large structural variations not only between metals but also between crystals grown from the same mother liquor. Large variations are observed in the M–N–C­(ring) angles that lead to close metal ring carbon distances [105.6(1)–115.7(2)°] as well as in the M–N–(NC4Me4 ring centroid) angles (152.2–167.3°). The synthesis and structure of 4d, 4f, and 5f metal examples are described, and the results are compared to predictions from the density functional theory. The reasons for the variable structures displayed by the (C5Me5)2M­(NC4Me4) complexes are discussed.