p-tert-Butylcalix[4]arene, [Calix(But)(OH)(4)], reacts with Mo(PMe3)(6) and W(PMe3)(4)(eta(2)-CH2PMe2)H to yield compounds of composition {[Calix(But)(OH)(2)(O)(2)]M(PMe3)(3)H-2} which exhibit unprecedented use of a C-H bond of a calixarene methylene group as a binding functionality in the form of agostic and alkyl hydride derivatives. Thus, X-ray diffraction studies demonstrate that, in the solid state, the molybdenum complex [Calix(But)(OH)(2)(O)(2)]Mo(PMe3)(3)H-2 exists as an agostic derivative with a Mo center dot center dot center dot H-C interaction, whereas the tungsten complex exists as a metallated trihydride [Calix-H-But(OH)(2)(O)(2)]W(PMe3)(3)H-3. Solution H-1 NMR spectroscopic studies, however, provide evidence that [Calix-HBut(OH)(2)(O) (2)]W(PMe3)(3)H-3 is in equilibrium with its agostic isomer [CalixBut(OH)(2)(O)(2)] W(PMe3)(3)H-2. Dynamic NMR spectroscopy also indicates that the [M(PMe3) H-3(2)] fragments of both the molybdenum and tungsten complexes [Calix(But)(OH)(2)(O)(2)]M(PMe3)(3)H-2 migrate rapidly around the phenolic rim of the calixarene on the NMR time scale, an observation that is in accord with incorporation of deuterium into the methylene endo positions upon treatment of the isomeric mixture of [Calix(But)(OH)(2)(O)(2)]W(PMe3)(3)H-2 and [Calix-H-But(OH)(2)(O)(2)]W(PMe3)(3)H-3 with D-2. Treatment of {[Calix(But)(OH)(2)(O)(2)]W(PMe3)(3)H-2} with Ph2C2 gives the alkylidene complex [Calix(But)(O)4]W=C(Ph)Ar[Ar = PhCC(Ph)CH2Ph].