The reactivity and thermal stability of zirconium and hafnium complexes containing the N-alkyl-substituted amine biphenolate ligands of the type [RN(CH2-2-O-3,5-C6H2(tBu)(2))(2)](2-) ([R-ONO](2-); R = tBu (1a), iPr (1b), or nPr (1c)) were investigated. The reactions of either [1a]M(OiPr)(2) (M = Zr or Hf) with equimolar H-2[1a] or M(OiPr)(4)(HOiPr) (M = Zr or Hf) with 2 equiv of H-2[1a] at 25 degrees C in diethyl ether or 80 degrees C in toluene afford moderate yields of colorless crystals of M[1a](OiPr)-(iPrOCH(2)-2-O-3,5-C6H2(tBu)(2)) (M = Zr (4a) or Hf (5a)). Controlled experiments revealed that the Production of 4a and 5a proceeds via unexpected thermal degradation of H-2[1a] that produces a highly reactive, transient ortho-quinone methide intermediate. Similar reactions employing H-2[1b] and H-2[1c], however, led to the formation of homoleptic bis-ligand complexes Zr[1b](2) (8b) and M[1c](2) (M = Zr (8c) or Hf (9c)) as colorless crystals. Decisive factors governing these divergent reaction pathways and complex constitutions are discussed. The X-ray structures of 4a, 5a, 8b, 8c, and 9c are presented.