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Titanium(II) and Titanium(III) Tetrahydroborates. Crystal Structures of [Li(Et2O)2][Ti2(BH4)5(PMe2Ph)4], Ti(BH4)3(PMe2Ph)2, and Ti(BH4)3(PEt3)2

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posted on 06.02.2006, 00:00 by Dean M. Goedde, Gregory S. Girolami
The molecular structures of the titanium(III) borohydride complexes Ti(BH4)3(PEt3)2 and Ti(BH4)3(PMe2Ph)2 have been determined. If the BH4 groups are considered to occupy one coordination site, both complexes adopt distorted trigonal bipyramidal structures with the phosphines in the axial sites; the P−Ti−P angles deviate significantly from linearity and are near 156°. In both compounds, two of the three BH4 groups are bidentate and one is tridentate. The deduced structures differ from the one previously described for the PMe3 analogue Ti(BH4)3(PMe3)2, in which two of the tetrahydroborate groups were thought to be bound to the metal in an unusual “side-on” (η2-B,H) fashion. Because the PMe3, PEt3, and PMe2Ph complexes have nearly identical IR spectra, they most likely have similar structures. The current evidence strongly suggests that the earlier crystal structure of Ti(BH4)3(PMe3)2 was incorrectly interpreted and that these complexes all adopt structures in which two of the BH4 groups are bidentate and one is tridentate. The synthesis of the titanium(III) complex Ti(BH4)3(PMe2Ph)2 affords small amounts of a second product:  the titanium(II) complex [Li(Et2O)2][Ti2(BH4)5(PMe2Ph)4]. The [Ti2(BH4)5(PMe2Ph)4]- anion consists of two Ti(η2-BH4)2(PMe2Ph)2 centers linked by a bridging η22-BH4 group that forms a Ti···(μ-B)···Ti angle of 169.9(3)°. Unlike the distorted trigonal bipyramidal geometries seen for the titanium(III) complexes, the metal centers in this titanium(II) species each adopt nearly ideal tbp geometries with P−Ti−P angles of 172−176°. All three BH4 groups around each Ti atom are bidentate. One of the BH4 groups on each Ti center bridges between Ti and an ether-coordinated Li cation, again in an η22 fashion. The relationships between the electronic structures and the molecular structures of all these titanium complexes are briefly discussed.