Tautomerism in organometallic compounds

8-23 T of relative area 1 1 2 appear, consistent with a structure in which the iron atom is co-ordinated to one of the double bonds in the allene. At room temperature the iron is undergoing motion relative to the n electron systems of the allene, as indicated by the dotted lines in (81). 

Equilibria of a different type are possible in some cases in n allyl complexes. The authors of the last paper distinguish three types of a and n allyl complexes and detail the types of n.m.r. spectra expected from each of them these complexes are shown in the structures (82-84). In the last category falls tetra-allylzirconium. At — 10°C, this compound shows a spectrum of the AX4 type, with all four protons of the CH2 groups in an allyl ligand equivalent on a time-average basis, and at — 74 C the spectrum is of the A2M2X type expected for one of the tautomers of the equilibrium (84). See also Cotton et al  

In the case of the bisallylrhodium chloride dimer, which at 140°C has an AX4 type spectrum, but has at — 20°C two pairs of dissimilar allyl groups [two 7 bonded like (82) and two others, presumably n bonded like (83)] i.e., structure (85), it is suggested that the two types of allyl groups exchange their roles at high temperatures.  

A different type of duality of ligand function from that occurring in system (85) is observed in (x-cyclopentadienyl 7t-cyclopenta-dienyl complexes, e.g., ((T-C5H5)Cr(NO)2(7t-C5H5) and the 

Valence tautomerism is invoked to account for the temperature variation of the n.m.r. spectrum of a-methyloxepine (87), and of 3-methoxy-4-azatricyclo[.3.3.2.0 ]deca-3,6,9-triene (methoxy-azabullvalene).  

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