Hydride complex, high coordinate

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

Numerous phosphine and arsine complexes have been synthesized and characterized predominately with osmium in the +2, + 3 or +4 oxidation states. Examples include [OsCl2(dppm)2] [108341-10-2], [OsC13(P(CH3)2(C6H5)3] [20500-70-3], [0s2Cl6(dppm)2(0)] [87883-12-3], and [Os(AsC2Hb(C6Hb)2)4H2] [27498-19-7]. An example of an unusually low oxidation state is the Os(—2) complex K2[Os(PF3)4] [26876-74-4]. High coordination numbers and formal oxidation states are found in the phosphine hydrides, eg, [Os(P(CH3)(C(5HB)2)H6] [25895-55-0] and... 

In the majority of its metal complexes, the hydride ligand normally functions in the /x1 (terminal), /tx2(edge-bridging), and /x3 (triangular face-capping) modes. Research efforts in recent years have led to the syntheses of an increasing number of high-coordinate (/u4, /u5, and /x6) hydride complexes. 

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