Hydridic dimeric complexes

The most general route to anionic or hydridic dimeric complexes utilizes a metal carbonyl dimer as a starting complex. Reaction of Mn2(CO)io with NaBH4 and subsequent treatment with phosphoric acid led to H2Mn2(CO)9F... 

An important contribution to an understanding of the mechanism of propene dimerization has been obtained by studying the reaction of nickel-hydride model complexes (55). The formation of the propyl- and isopropyl-nickel complexes 48 and 49 has been observed in the reactions of HNi(PR3)Cl complexes (50) with propene at -78°C [Eq. (15)] ... 

The dimeric complex [RhClL2 L>, present in varying amounts according to the conditions, is also an effective catalyst via a similar hydride route involving complex 1. An originally proposed (80) dimeric tetrahydride was not detected. Detailed crystal structures of both the red and orange forms of RhCl(PPh3)3 have appeared (81). 

The resting state of the propanoate catalysts may well be an acyl complex [60,61], while the attack of alcohol at the acylpalladium complex is considered to be the rate-determining step. It is probably more precise to say that fast preequilibria exist between the acyl complex and other complexes en route to it and that the highest barrier is formed by the reaction of alcohol and acylpalladium complex. The precise course of the reaction is still not known presumably deprotonation of the coordinating alcohol and the migratory elimination are concerted processes, accelerated by the steric bulk of the bidentate ligand. Toth and Elsevier showed that the reaction of an acetylpalladium complex and sodium methoxide is very fast and occurs already at low temperature to give methyl acetate and a palladium(I) hydride dimer [46]. 

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

Complex hydrides give a variety of interesting structures. For LiBH4 derivatives, a monomeric example crystallized from THF " shows three p B-H Li bridges and a very short B-Li distance of 2.19 A (cf. C-Li, 2.36 A in the analogous (/u-H)3C environment of solid (MeLi)co). However, when TMEDA is used as the donor in the parent borohydride, a dimeric complex (16) results, containing four p and two p B-H Li contacts. A tetrameric complex with p ... 

Synthesis and reactivity of the yttrium hydride dimer [Cp Y(OAr)(/i-Fl)]2 (Ar = 2,6-Bu2C6H3) have been investigated in detail.294 295 Synthetic routes leading to tetranuclear yttrium and lutetium hydride clusters have been worked out. The complexes are accessible by reacting precursors of the type (CsMe4SiMe2R)Ln(CFI2SiMe3)2(TF[F)... 

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