RhH2

RhH2(PhNNNPh)(PPh3)2. 4,1022 RhC48H44P4 [Rh(PHPh2)4]+, 4, 925 RhC48H52I2P 3S2... 

The reaction of silica-supported bis(allyl)Rh complexes with PMe3 followed by hydrogen treatment leads to the synthesis of the surface SiO-RhH2(PMe3)4+ as the first example of a cationic organometallic complex attached to the silica surface by ion pairing. The counterion is presumed to be a siloxy group SiO- on the silica surface.276 277... 

RhHI 3 (L=P(i-Pr)3) is capable of forming the water adduct (21). The water addition to RhHL3 takes place in pyridine readily at room temperature to give an ionic product [RhH2(py)2L2]0H (eq. 17), which can be isolated as its BPh salt (v(Rh-H) 2076, 2112... 

A number of other elemental reactions for H2 generation are conceivable, if C02 is accumulated in the reaction system. For example, the viable intermediate, RhH(C0)L2 should react with H2C03 to give Rh(CO)(0C02H)L2 (v(C0) 1952 v(C=0) 1615 cm-1) via postulated species RhH2(C0)(0C02H)L2 (eq. 27). 

The synthesis of cationic rhodium complexes constitutes another important contribution of the late 1960s. The preparation of cationic complexes of formula [Rh(diene)(PR3)2]+ was reported by several laboratories in the period 1968-1970 [17, 18]. Osborn and coworkers made the important discovery that these complexes, when treated with molecular hydrogen, yield [RhH2(PR3)2(S)2]+ (S = sol-vent). These rhodium(III) complexes function as homogeneous hydrogenation catalysts under mild conditions for the reduction of alkenes, dienes, alkynes, and ketones [17, 19]. Related complexes with chiral diphosphines have been very important in modern enantioselective catalytic hydrogenations (see Section 1.1.6). 

The water-soluble analogue of Wilkinson s catalyst, [RhCl(TPPMS)3] [TPPMS = PPh2(C6H4S03Na)], prepared in situ from Rh(/<-Cl)(diene)]2 and TPPMS, reacts with hydrogen in aqueous solution to yield [RhH(TPPMS)3], instead of [RhH2(TPPMS)3], according to Eq. (6) ... 

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