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The synthesis, properties, and reactivity of monocyclopentadienyltitanium(III) complexes stabilized by hydridoborato (BH3R)(-) ligands are reported. The reactions of the titanium(IV) trichlorides [Ti(eta(5)-C5H5-nMen)Cl-3] with excess LiBH3R (R = H, Me) afford dimeric [{Ti(eta(5)-C5H5-nMen)(BH4)(mu-BH4)}(2)] (n = 5 (1), 4 (2), 0 (3)) or monomeric [Ti(eta(5)-C5Me5)(BH3Me)(2)] (4) titanium(III) derivatives. Compound 3 is a thermally unstable oil and decomposes at room temperature forming a mixedvalence Ti-II/Ti-III tetrametallic species [{Ti(eta(5)-C5H5)(BH4)}(2){(mu(3)-B2H6)Ti(eta(5)-C5H5)}(2)] (5) with dianionic diborane(6) ligands in a rare mu(3)-kappa(2):kappa(2):kappa(2)-B2H6 coordination mode. Density functional theory (DFT) calculations for 5 agree with a triplet ground state with two inner titanium(II) atoms bonded by a single bond and two outer titanium(III) centers. In contrast to 3, the monomeric tetrahydrofuran adduct derivative [Ti(eta(5)-C5H5)(BH4)(2)(thf)] (6) is a stable solid. Dimeric 1-3 react with 2,6-lutidinium salts (LutH)X (X = BPh4, OSO2CF3) to afford ion pairs, [Ti(eta(5)-C5H5-nMen)(BH4)L-2](BPh4) (L = thf, n = 5 (7), 4 (8), 0 (9); L = py, n = 5 (10)), or molecular aggregates, [{Ti(eta(5)-C5Me5)(BH4)(mu-O2SOCF3)}(2)] (11) and [{Ti(eta 5C5H5-nMen)(mu-O2SOCF3)(2)} x] (n = 5, x = 3 (12); n = 4, x = 4 (13)), via formation of H-2 and (Lut)BH3. Diamagnetic titanium(II) derivatives [{Ti(eta(5)-C5Me5)(mu-H)}(2){(mu-H)(2)Al(BH3R)(thf)}(2)] (R = H (14), Me (15)) were isolated in reactions of complexes 1 and 4 with LiAlH4 in tetrahydrofuran., Spanish Structures of Excellence María de Maeztu, Ministerio de Ciencia, Innovación y Universidades, Universidad de Alcalá