The limits of steric crowding in tris(pentamethylcyclopentadienyl) complexes have been expanded by isolation of the first compound with this ligand set and two additional ligands, (C5Me5)(3)ML2. This result was obtained in a study of the effect of added ligands on the reactivity of sterically crowded (C5Me5)(3)La, 1. Although neither THF nor Ph3PO formed crystallographically characterizable (C5Me5)(3)LaLx complexes with 1, these (C5Me5)(3)La/L-x mixtures displayed enhanced reactivity compared to 1 with substrates such as C8H8 and CO2. Attempts to use adamantyl azide, AdN(3), to make (C5Me5)(3)LaLx complexes led to the first example of azide insertion into a metal cyclopentadienyl linkage to generate (C5Me5)(2)La[eta(2)-(N,N')-(C5Me5)NN'N '' Ad](N(3)Ad). Addition of Me3CCN to 1 produced the first crystallographically characterized (C5Me5)(3)ML2 complex, (C5Me5)(3)La(NCCMe3)(2).