The thermal decomposition of group 4 trimethylsilylmethyl derivatives [M(eta(5)-C5Me5)(CH2SiMe3)(3)] (M = Ti (1), Zr (2), Hf (3)) in solution and their reactivity with N,N-dimethylamine-borane were investigated. Heating of hydrocarbon solutions of compounds 2 and 3 at 130-200 degrees C results in the elimination of SiMe4 and the clean formation of the singular alkylidene-alkylidyne zirconium and hafnium compounds [{M(eta(5)-C5Me5)}(3){(mu-CH)(3)SiMe}(mu(3)-CSiMe3)] (M = Zr (4), Hf (5)). The reaction of 2 and 3 with NHMe2BH3 (>= 1 equiv) at room temperature affords the dialkyl(dimethylamidbborane) complexes [M(eta(5)-C5Me5)(CH2SiMe3)(2)(NMe2BH3)] (M = Zr (6), Hf (7)). Compounds 6 and 7 are unstable in solution and decompose with formation of the alkyl(dimethylamino)borane [B(CH2SiMe3)H(NMe2)] (8), SiMe4, and other minor byproducts, including the tetranuclear zirconium(III) octahydride complex [{Zr(eta(5)-C5Me5)}(4)(mu-H)(8)] (9) in the decomposition of 6. Addition of NHMe2BH3 to the titanium tris(trimethylsilylmethyl) derivative 1 gives the trinuclear mixed valence Ti(II)/Ti(III) tetrahydride complex [{Ti(eta(5)-C5Me5)(mu(3)-H)}(3)(mu(3)-H)(mu(3)-NMe2BH2)] (10) at 45-65 degrees C. While the complete conversion of 1 under argon atmosphere requires excess NHMe2BH3 (up to 15 equiv), complex 10 is readily prepared with 3 equiv of NHMe2BH3 under a hydrogen atmosphere indicating that the formation of 10 involves hydrogenolysis of 1 in the presence of (NMe2BH2)(2). In absence of amine-borane, the reaction of 1 with H-2 leads to the tetranuclear titanium(III) octahydride [{Ti(eta(5)-C5Me5)}(4)(mu-H)(8)] (11), which upon addition of NHMe2BH3 and subsequent heating at 65 degrees C affords complex 10. The X-ray crystal structures of 2, 4, 5, 10, and 11 were determined.