Octahedral complexes trans effect

Pt(II) metal complexes, are known to exhibit a structural trans effect. In octahedral complexes, trans effects can also occur, even if these effects are less pronounced... 

Where unidentate ligands are present, the ability to effect the resolution of an octahedral complex (i.e. to separate 2 optical isomers) is proof that the 2 ligands are cis to each other. Resolution of [PtCl2(en)2] therefore shows it to be cis while of the 2 known geometrical isomers of [CrCl2en(NH3)2] the one which can be resolved must have the cis-cis structure since the trans form would give a superimposable, and therefore identical, mirror image ... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

As the trans effect theory indicates, there should be some relationship between lability of a ligand and its role as a labilizing group in another position in a complex. In an octahedral complex reacting via a dissociative mode of activation, the transition state has five strongly bound ligands. This state will be stabilized... 

At this point, it is appropriate to mention some of the evidence that indicates a trans effect in octahedral complexes, but only a brief description will be given. In the reaction of Mo(CO)6 with pyridine, only three CO ligands undergo replacement, and the product has the structure... 

In addition to the indications of an octahedral trans effect presented, there exists structural information in the form of bond lengths and spectral data similar to that described earlier for square planar complexes. Although the trans effect in octahedral complexes is not the dominant influence that it is in square planar complexes, there is no doubt that there is such an effect. 

As happens for other physico-chemical techniques, one must first ask if an electrochemical investigation is able to distinguish geometric isomers of the type cisjtrans or facjmer metal complexes. In principle, this is possible since, as mentioned previously, the redox potential of an electron transfer is influenced also by steric effects. For instance, we have seen in Chapter 5 that some octahedral complexes of the scorpiand diammac display different electrochemical responses, depending on whether the two outer amino groups assume cis or trans arrangements. One must keep in mind, however, that the differences in the electrochemical response of isomeric complexes can sometimes be quite small, so may escape a first examination. 

Gs, trans, or metal effects occur when, e.g., an octahedral coordination compound ML6 [7] is transformed into a new complex, either MLSX [2], where a ligand has been altered, or M L6 [5], where the metal M has been replaced by an isovalent metal M. The comparison of either [2] with [7], or [2] with [7], discloses either cis and trans effects or metal effects inasmuch as the properties of the ligands L are altered by this variation in the first case, the substituent effect of X is subdivided into the cis and trans effects. 

For a recent review on trans-effects in octahedral transition metal complexes, see ... 

Elder, R. C., Heeg, M. J., Payne, M. D., Trkula, M., and Deutsch, E. (1978). trans Effect in octahedral complexes. 3. Comparison of kinetic and structural trans effects induced by coordinated sulfur in sulfito- and sulfinatopentaamminecobalt(III) complexes. Inorg. Chem. 17,431-440. 

To overcome this problem we switched to [Re(CO)5Br] as the metal precursor, because it is one of the few octahedral complexes that undergoes an exclusive cis ligand exchange, because of the remarkable trans effect of the carbonyl ligands. The replacement of two equatorial cis-CO groups gave the desired neutral... 

Similar effects are observed with octahedral complexes where JciB 1.5 /trans as illustrated in structures VI and VII (46). The ratio of the... 

---