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The synthesis and characterization of group 4 permethylpentalene (Pn* = C8Me6) hydride complexes are explored; in all cases, multimetallic hydride clusters were obtained. Group 4 lithium metal hydride clusters were obtained when reacting the metal dihalides with hydride transfer reagents such as LiAlH4, and these species featured an unusual hexagonal bipyramidal structural motif. Only the zirconium analogue was found to undergo hydride exchange in the presence of deuterium. In contrast, a trimetallic titanium hydride cluster was isolated on reaction of the titanium dialkyl with hydrogen. This diamagnetic, mixed valence species was characterized in the solid state, as well as by solution electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The structure was further probed and corroborated by density functional theory calculations, which illustrated the formation of a metal-cluster bonding orbital responsible for the diamagnetism of the complex. These permethylpentalene hydride complexes have divergent structural motifs and reactivity in comparison with related classical cyclopentadienyl analogues.

A ‘frustrated Lewis pair’-type mechanism allows the first observation of reversible H2 addition to a single-site main-group complex., A range of amines catalyse the oxidative addition (OA) of H2 to [(Me3Si)2CH]2Sn (1), forming [(Me3Si)2CH]2SnH2 (2). Experimental and computational studies point to ‘frustrated Lewis pair’ mechanisms in which 1 acts as a Lewis acid and involve unusual late transition states; this is supported by the observation of a kinetic isotope effect for Et3N. When DBU is used the energetics of H2 activation are altered, allowing an equilibrium between 1, 2 and adduct [1·DBU] to be established, thus demonstrating reversible oxidative addition/reductive elimination (RE) of H2 at a single main group centre.

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