Scheme I b' a'(diphos)Ir(CH3) and for (C8H12)[(C6H5)(CH3)2P]2Ir(CH3) are entirely consonant with the mechanism suggested by Shapley and Osborn (see Scheme I) in which there are successive "pseudorotations" about Pt, R, and P2. For equilibration of vinyls 1 and 2 to occur, the reaction must proceed from ground state a, through the high-energy intermediates b and b', back to ground state a'. In the high-energy inter-mediate, a methyl group occupies an equatorial position and the two phosphine groups occupy an equatorial and an axial position. Since ground state a is of higher energy for the diphos compound, due to extra strain associated with a small equatorial P-Ir-P' angle, the diphos compound is expected (as observed) to undergo the intramolecular interconversion a -* a' at a greater rate than the analogous bis-(monodentate)phosphine complex in which the P-Ir-P' angle is not strained by chelation and is closer to the ideal equatorial value of 120°.Further information on these systems is expected to be obtained from current X-ray structural analyses on the species (C8H12)[( C6Hs)2P(CH2)nP(C6H5)2]Ir(CH3) in which n has values other than 2. (The case with n = 2, of course, is covered by the present article.) Registry No. (C8H12)(diphos)Ir(CH3), 36974-34-2. Acknowledgments. S. A. B. gratefully acknowledges the receipt of an NSF predoctoral fellowship. This work was generously supported by the National Science Foundation through Grants GP-26293 and GP-33018 (to M. R. C.).

The title compound crystallizes in space group P1 with a = 7.7662 (6)

Sir:Yellow or brown diamagnetic planar and blue-violet paramagnetic octahedral are both well-established forms for nickel(I1). With certain polyamine complexes, both conformations exist in solution in detectable concentrations. Thus, blue aqueous solutions containing Ni(trien)(HzO)z2+, Ni(en)n(HzO)z2+, or Ni(pn)z-(Hz0)22+ turn brownish yellow on heating. The absorption band at -440 mp characteristic of tetragonal nickel(I1) is developed, and the original color returns on cooling the so1ution.ls2 Aqueous solutions of the nickel complex of the quadridentate ligand NH2(CH&-NH(CH2)3NH(CH2)2NH2 (abbreviated 2,3,2-tet) are already brown3 and thus contain substantial amounts of the planar form at room t e m p e r a t~r e .~ Information on the rate at which such equilibria as (L = trien, or 2,3,2-t et) octahedral-Ni(L)(HzO)?2f e planar-Ni(L)z-+ 2 H i 0 are established is lacking, but the system is amenable to the relaxation method of rate measurement.j Sizable absorbance changes at 440 mp with a small temperature rise, 2.5-6.5 O, particularly with the 2,3,2-tet complex, allowed work at low amplification and thus at the shortest time ranges of the temperature-jump apparatus.j It was found, however, that with the 1 : l nickel(I1)-trien and nickel(II)-2,3,2-tet systems, the establishment of the yellow-blue equilibria was complete at 5" within 10 psec which was the shortest time we could reach with our equipment. The first-order rate constant for the conformation change must be >lo5 sec-l. Only a slight "electronic ripple" was observed when solutions of Ni(tren)(H.O)?*f or K N 0 3 were used in the temperature-jump cell. The amine tren, N(CH2CH2NH2)3, can only coordinate to leave two cis positions in the octahedron.We believe that with 2,3,2-tet the fast transition observed is between the Trans-octahedral and the planar complex. Of relevance are the reactions of the complex with EDTA and oxalate which indicate that there are at least two blue isomers of Ni(2,3,2-tet)(H20)22+ present in solution. Upon mixing EDTA and Ni-2,3,2-tet (1 : l ) complex there is an absorbance jump at 380 mp corresponding to about 40% of the total complex present. the remainder, both blue and yellow forms, reacting by a slow first-order process (k = 9.3 X sec-l at pH 8.34).6 The fast transition with the trien complex may be due to a trans-octahedral-planar equilibrium although Ni(trien)(H20)2?+ is believed to have predominantly the cis-octahedral structure in aqueous(1) C. I<. Jprgensen, Acta Chem. aqueous solution (25", p = 0.5 M ) the yellow form of Ni (2,3,2-tet)Z+ cannot be more than 25 % of the total Ni(2,3,2-tet)?' as opposed to 50% suggested in a n earlier report.3 This estimate is based on spectral measurements, using a n approximate value for the molar absorptivity of the yellow form at 440 m p of 67 (obtained in high electrolyte concentration, 7.4 M NaCIO1). The planar yellow form is at most 2 ' % of the total Ni(trien)'+ under the conditions p = 0.5 M , 25 ^. ( 5 ) M . Eigen and L. work in progress.solution. The high rate constant resembles...

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