Protonation of the title complexes leads to either (q*-H2) or trans-dihydride monocations depending on the length of the chain connecting the two phosphine atoms; TI and JHD measurements are used to support these structures.We reported recently1 that protonation of [(q5-C5H5)Ru(PPh3)(ButNC)H] gave the (q2-H2) cation, [(775-

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Protonation of [(q5-C5H,)Ru(PPh3)(CNBut)H] with HPF6 gives the title complex, a cationic molecular hydrogen complex.Recently reported molecular hydrogen complexes fall into two classes: neutral complexes formed by reaction of molecular dihydrogen with co-ordinatively unsaturated precursors,1 and cationic complexes containing both metal hydride and metal-(q2-H2) ligands, formed by protonation of metal polyhydrides,2J or by reaction of molecular dihydrogen with a cationic monohydride.4 We now report the preparation of a cationic complex containing only a metal-(y2-H2) ligand from protonation of a neutral monohydride precursor.The monohydride, [ (+C5H5)Ru(PPh3)(CNBut)H] (1) was prepared by the reaction of LiA1H4 on [(q5-C5H5)Ru(PPh3)(CNBut)C1]5 as sand coloured crystals in 94% yield. The compound was characterised by elemental analyses and i.r. [CH2C12, vmax,: 2049, (CN); 1974, (Ru-H) cm-11 and 1H n.m.r. spectroscopy [C6D6, 6: -11.4 (d, JpH 34 Hz,Reaction of (1) with aqueous HPF6 in dichloromethane solution at 0 "C caused an immediate colour change and from the orange solution the pale yellow cationic complex, [(y5-

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