and methyl methacrylate are not. Further experiments will show if these substrates will react under more forcing conditions or with more effective H-transfer reagents. Phenalene, known to be an especially reactive H donor, was used successfully .C2 '1This uncdtalyzed H-transfer reaction with its unequivocal, undisturbed reaction sequence is a good subject for further investigation of additional mechanistic details of the bimolecular radical formation by hydrogen transfer. The structure-reactivity relationships may then be better understood.

The reversible formation and cleavage of a metal-metal bond is a key reaction in dinuclear and cluster chemistry.1 A few cases have been reported in which such a process is accompanied by the reversible addition of two electron donors such as CO or PR,.1 However, to our knowledge there have, so far, been no well characterized examples of the complete cleavage of a metal-metal bond accompanied by the reversible addition of H2 under mild conditions. There are relatively few examples of the reversible addition of H2 to a metal cluster2 (see Note Added in Proof, ref 10), and in the cases where the products have been fully characterized, no cleavage of metal-metal bonds occurs.2-3 We report here the first example of the facile, reversible addition of H2 to a metal cluster which is accompanied by a clearly recognized reversible metal-metal bond cleavage.Reaction of di-rezf-butylphosphine (f-Bu2PH) with Ru,(CO),2 in di-n-butylether (0.5 h) at 100 °C gives the purple bis-phosphido bridged trinuclear complex Ru3(CO)g(*i-H)2(»i-f-Bu2P)2 (1) in 55% yield.4 The diphenylphosphido analogue of 1 has been reported, and the two compounds have similar structures in the solid state5 (see below). Spectroscopic data for 1 (1R, 'H, 3IP, NMR) is in accord with the solid-state structure as determined by X-ray crystallography.6 The IR spectrum contains only terminal i>co absorptions, while the H NMR shows a triplet at h -17.19 (7p-H = 19.5 Hz) assigned to two equivalent hydride ligands. A similar resonance is observed in the spectrum of Ru3(CO)8(3x-H)2(3t-Ph2P)2, and the triplet pattern can be attributed to an AA'XX' type of spectrum.5 The 3IP|'H| NMR spectrum of 1 is a singlet at 6 256.05 consistent with phosphido groups bridging metal-metal bonds.7Purple hexane solutions of 1 rapidiy turn orange when exposed to H2 (1 atm) at ambient temperature. From these solutions orange crystals of the 50-electron complex Ru3(CO)g(3t-H)2-(H)2(m-J-Bu2P)2 (2) may be isolated by crystallization from hexane under a hydrogen atmosphere. This compound is stable with respect to loss of H2 in the solid state either under vacuum or a

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