The reaction of 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) with [La{Al(CH₃)₄}₃] resulted in C–H activation, leading to the formation of [(TMTAC)La{Al(CH₃)₄}{(μ₃-CH₂)[Al(CH₃)₂(μ₂-CH₃)]₂}] (1) containing a bis(aluminate) dianion and subsequent extrusion of methane. A similar reaction with [Y{Al(CH₃)₄}₃] led to the formation of CH₄, [TMTAC{Al(CH₃)₃}₂] (2) and {[(TMTAC)Y][Y₂(μ₂-CH₃)][{(μ₆-C)[Al(μ₂-CH₃)₂(CH₃)]₃}{(μ₃-CH₂)(μ₂-CH₃)Al(CH₃)₂}₂] (3), containing a six-coordinate carbide ion and two [CH₂Al(CH₃)₃]₂ ⁻ anions. Compound 3 is a product of multiple C–H activation. This reaction was monitored by in situ¹H NMR spectroscopy. The analogous reaction with [Sm{Al(CH₃)₄}₃] led to the formation of 2, of [(TMTAC)Sm{(μ₂-CH₃)(CH₃)₂Al}₂{(μ₃-CH₂)₂Al(CH₃)₂}₂] (4), which contains a tris(aluminate) trianion, and [{(TMTAC)Sm}{Sm₂(μ₂-CH₃)}{(μ₆-C)[Al(μ₂-CH₃)₂(CH₃)]₃}{(μ₃-CH₂)(μ₂-CH₃)Al(CH₃)₂}₂] (5), which is isostructural to 3. The products were characterised by elemental analyses (except 4, 5), 1 by multinuclear NMR spectroscopy and compounds 1, 2, 3, 4 and 5 by X-ray crystallography. Quantumchemical calculations were undertaken to support the crystallographic data analysis and confirm the structure of 3 and to compare it with an analogous compound where the central six-coordinate carbon has been replaced by oxygen. The investigations point to a mechanism of sterically induced condensation of [Al(CH₃)₄]⁻groups in close proximity in the coordination spheres of the rare-earth metal atoms, which is dependent on the size of these metal atoms.