The 31P resonance of the trans form of 2 appears to show line broadening effects even when the cis isomer is in the slow-exchange limit (Figure 2). It is probable that the energy minimum for the trans molecule corresponds to the distorted structure indicated in Figure 3a and that the inequivalent phosphorus ligands are undergoing rapid intramolecular exchange.The passage of the cis and trans resonances in 2 to a simple quintet with increasing temperature shows that cis-trans isomerization via a polytopal1-4 rearrangement is operative here. The analysis of the hydride spectrum for compound 1 is the same as for 2 except that only the cis isomer was detected.The distortions in the cis and trans isomers indicated in Figure 3 suggest a polytopal rearrangement alternative to the trigonal or "Bailar" twist.5 The distortion modes could, in highly excited vibrational states, approach a tetrahedral disposition of phosphorus nuclei about the iron nucleus; hydrogen traverse (tunneling or classical) of trigonal faces would then complete the nuclear permutation process. This alternative is more attractive on steric grounds because of the obvious steric relief in the transition state. We cannot now rigorously distinguish between these two possible rearrangements, but detailed analysis of nmr transitional line shapes using the density matrix approach and analogous studies of other L4MH2 complexes now in progress may provide a definitive answer.Full details of spectral assignments and mechanistic studies will be published shortly.

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