Reduction of ( q5-C5H5) R u ( P P ~~) ~C I(2) with LiAIH4 in tetrahydrofuran leads to the trihydride ( 75-C5H5) Ru( PPh3)H (1) together with, as a minor product, the expected monohydride ( Y ~-C ~H ~) R U ( P P ~~) ~H(3) which can be prepared efficiently by treatment of the chloride (2) with NaOMe; reduction of ( q5-C5H5) Ru( Ph2PCH2CH2PPh2)CI ( 4) with LiAlH4 gives only the monohydride ( q5-C5H5)Ru-(Ph2PCH2CH2PPhZ)H (5).

The use of long range H-31 P spin-spin coupling constants as criteria for the assignment of the stereochemistry of 5-substituted q4-cyclopentadiene transition-metal complexes is described.Nucleophilic addition to cyclopentadienyl ligands bonded to 18-electron organotransition-metal cations is generally accepted to be stereospecific with the nucleophile approaching the unco-ordinated face of the cyclopentadienyl ligand to yield exo 5-substituted q4-cyclopentadiene Complexes.' This is consistent with these addition reactions involving SN2 displacement of the metal-carbon bond by the nucleophile with the carbon atom being attacked undergoing inversion of configuration.2 The most common criterion for assigning the stereochemistry of 5-substituted q4-cyclopentadiene complexes has been the presence or absence of an i.r. absorption at ca. 2 750 cm-' attributed to C-Hex0.3 This absorption was however initially misassigned to C-Hendo and this has resulted in its subsequent history being rather confused.4 Recently serious doubt about the validity of this criterion has been expre~sed.~ We describe here the use of long range 1H-31P coupling constants to assign unambiguously the stereochemistry of such complexes.Long range spin-spin coupling between nuclei occurs over four bonds between atoms in a ' W ' arrangement.6 This is a well established phenomenon in hydrocarbon molecules but has not been extensively exploited in organometallic complexes. In cyclopentadiene complexes He,, but not Hendo is in the required geometric arrangement with any co-ordinated phosphorus atoms for such long range coupling to be expected, assuming rotation about the diene-metal bond. Examination of the 300-MHz n.m.r. spectra of a range of cyclopentadiene complexes containing phosphine ligands demonstrates that 1He,,-31P coupling can indeed be readily observed.

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Die Cyano‐ Komplexe (I) reagieren mit den entsprechenden Ru‐chloriden (II) zu den cyanoverbrückten Dirutheniumkomplexen (lIIa)‐(IIId).