A series of complexes of the type Rh(cyclam)X,+ and Rh(cyclam)XYn+ have been prepared and characterized, where cyclam represents 1,4,8,11-tetraazacyclotetradecane and X and Y represent OH-, H,O, CI-, Br-, I-, Ns-, NCS-, and NOz-. The infrared and electronic spectra are discussed with respect to assignment of the cis and trolls isomers, the linkage isomers, and the Rh-ligand stretching frequencies above 250 cm-'. The thermodynamic tratls effect is related to shifts in the Rh-ligand stretching frequencies. The intensities of the d-d transitions are related to distortion of the octahedral field to support the cis and tram assignments, and compared to show the decrease in bond constraint for propylene linkages in place of ethylene. Steric constraint accounts for the single case of stereoisomerization by cis-Rh(cyclam)12 +.

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The rates of replacement of X-from some trans-Rh en,LX+ complexes have been measured, where Land Xare chloride, bromide, or iodide. The rates are independent of the nature or concentration of the incoming nucleophile. When L-is chloride or bromide and X-is iodide the reactions proceed by a catalytic mechanism but in all other cases non-catalytic reactions occur unless certain catalytic reducing agents are added. When Xis chloride or bromide the trans-effect of L increases along the series CI, Br, I whether it is measured in terms of rate constants or enthalpies of activation. The activation enthalpies decrease by about 2 kcal./mole when L changes from chloride to bromide, or from bromide to iodide. The mechanism of the reactions is discussed in terms of the relative softness, or class (b) character, of the reactant complexes and the transition states. The results are consistent with a wide variation in the extent to which Rh-OH, bond-making participates in the transition state. It is concluded that in this case the energetics of the S , 1 and S, 2 reaction paths might be so similar as to make meaningless any precise assignment of mechanism. The high rrans-effect of iodide is ascribed to the ease with which it can compensate for either an increase or a decrease in electron-density at the rhodium.KINETIC studies of substitution reactions of octahedral complexes have been concentrated largely on cobalt (III), chromium(m), and, to a lesser extent, platinum(1v) systems.1 The reactions of cobalt (111) complexes have been studied mainly with the aim of determining the molecularity of the reactions, and the stereochemical rearrangements which occur have been a great help in this respect.2 Platinum(1v) complexes have been I (a) F.

Equilibrium constants have been obtained spectrophotometrically for the stepwise replacement by iodide of the chloride in trans-Rh en,CI,+, and of the bromide in trans-Rh en,Br,+.At 90' KI(Cl-:l-) = K,(Br-: I-) = 7-0. Extrapolation to 90" of data obtained a t lower temperatures gives K2(Cl-:l-) = 2-2 and &(Br-:I-) = 1.9. The temperature-dependence of the equilibrium constants lead to the following enthalpy data. AH,'(CI-: I-) = -0.1 f 0.9, AH20(Cl-:l--) = -6.2 f 9.4. AH<(Br-;l;) = -2.6 f 0.4, and AHso(Br-:l-) = 3.4 0.3 kcal./mole.From these, and other data obtained previously, it IS possible to obtain the relative enthalpies of bonding of the dihalogeno-complexes in aqueous solution. These are in the order di-iodo > iodobromo > iodochloro > dibromo > bromochloro > dichloro. The values are discussed in terms of class (a) or (b) character of these complexes and the extent to which this is affected by the nature of the halide in the trans-position.The preparation and ultraviolet and visible absorption spectra of some new trans-bisethylenediaminediacidorhodium(i1i) complexes are reported. PRECEDING Papers l v 2 in this Series have been concernedwith the ease with which one halide in a complex can be replaced by another, as indicated by the equilibrium constant for the interchange or, better, by the enthalpy change involved. We now describe further studies involving some trans-bisethylenediaminedihalogenocomplexes of rhodium(II1). These studies provide a measure of the class (a) or (b) ~h a r a c t e r , ~ or of the " hardness " or " softness," of the complexes involved, and thermodynamic effects transmitted from a halide across the rhodium to the trans-position have also been estimated. The measurements were made spectrophotometrically at constant ionic strength, and temperature variation was used to obtain enthalpy and entropy data. RESULTSUZtravioZet and Visible Absorption Spectra.-The main features of the spectra of the unmixed dihalogeno-complexes

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