Scandium and yttrium bis(ligand) mono(alkyl) complexes, of N-phenyl (L¹) N-2-isopropylphenyl (L²) and N-mesityl (L³) substituted ortho-tert-butylsalicylaldiminato ligands were prepared by alkane elimination from [M(CH₂SiMe₂R)₃(THF)₂] (R = Me, Ph) and two equivalents of proteo ligand (HL). The resulting [L₂M(THF)_n(CH₂SiMe₂R)] (n = 0–2, L = L¹ (1), L² (2), L³ (3)) complexes are thermally unstable, decomposing rapidly between −20 and 20 °C. In order to gain insight in to ligand features necessary to impart thermal stability in early transition metal organometallic chemistry, the decomposition pathways of 1–3 have been investigated and compared with more sterically congested N-2,6-diisopropylphenyl substituted analogues. Compounds 1 and 2 decompose rapidly and cleanly at room temperature by 1,3-migration of the entire CH₂SiMe₂R group to the aldimine carbon. By contrast, L³ mono(alkyls) 3 decompose cleanly by metallation of an ortho-C₆H₂Me₃ group. In the case of yttrium, the metallated alkyl undergoes subsequent 1,3-migration to the aldimine carbon, forming a five-membered C₄N-ring.