Polyhydrides of Sc, Zr and Hf and Their Proposed Formation.

Hydrogenation at 500 psi of (PNP)Sc(CH 3 ) 2 results in formation of a trinuclear polyhydride complex [(PNP)Sc] 3 (μ 2 ‐H) 4 (μ 3 ‐H) 2 ( 1 ) in 55 % yield. The solid‐state structure shows a non‐symmetric trinuclear species resulting from one pincer phosphine arm being demetallated, and where two hydrides bridge all three Sc centers, whereas the other four bridge two. Hydrogenation of (PNP)Zr(CH 3 ) 3 at 200 psi results instead in formation of a dinclear polyhydride species [(PNP)Zr(H)] 2 (μ 2 ‐H) 4 ( 2 ). Conducting the hydrogenation at atmospheric pressure, resulted instead in formation of the bridging methylidene complex [(PNP)Zr(CH 3 )] 2 (μ 2 ‐H) 2 (μ 2 ‐CH 2 ) ( 3 ), which cleanly converted to 2 , upon hydrogenation at higher pressure. Both 2 and 3 were also structurally characterized. Hydrogenation of (PNP)Hf(CH 3 ) 3 at 200 psi resulted in incomplete hydrogenation with some formation of dinuclear mono‐ and dimethyl‐polyhydride complexes [(PNP)Hf(CH 3 )] 2 (μ 2 ‐H) 4 ( 4 ) and [(PNP)Hf(CH 3 )][(PNP)Hf(H)](μ 2 ‐H) 4 ( 5 ), which were identified by solid‐state X‐ray structural studies. Based on these results, we propose a pathway for the complete hydrogenation of (PNP)Zr(CH 3 ) 3 to 2 .