Correlations between polarographic and spectroscopic parameters of some diacidotetramminerhodium (III) complexes

INORG. NUCL.

CHEM. LETTERS Vol.

S, pp.

291.293, 1%9.

OF SOME D I A C I D O T E / R A ~ O D I U M

Pe,gemon P r e . .

Printed

In

Great

Britain

(III) COMPLEXES.

By D.R. Crow Department of Chemistry, The Woolwioh Polytechnic, London, England. (Received 23 December 1968)

A comparison has been made of the polarographic and spectroscopic characteristics of a number of rhodium (IIl) complexes of the type [Rh en 2 X2] n+

which were prepared by the method described by Johnson

and Basolo (I). Polsrograms were plotted manually and spectra recorded on a Unicam S.P.800 U.V. Spectrophotometer. Solutions used for polarographic measurements were 0.5 mN in each complex and 0.I M in soditm perchlorate as supporting electrolyte. In common with many other rhodium reductions, the waves tended to be obscured by large maxima and 0.1% gelatin was needed for their elimination. An earlier report of polarographic and spectroscopic correlations, for inert transition metal complexes, was made by Vl~ek (2) who showed that, for complexes of the type [GoIII (NH3)5 X I n+, the half-wave potential is a function of the d-orbital splitting induced by ligand X. In the present study it was found that a linear correlation exists between half-wave potential and the first d-d spectral band position. In Fig.l. the various half-wave potentials are plotted as a function of the position of the first absorption band of the complexes relative to that of the "parent" complex [Rh en3 ]5+ . Replacement of one ethylenediamine gr°up by two acido-groups shifts the reduction potential towards more positive values to an extent dependent upon the separation of the parent and heteroligands in the Spectroohemical

Series.

In all cases, two-electron irreversible reductions were 291

DIACIDOTETRAMMIHERHODIUM

292

(111) COMPLEXES

Vol. 5, 14o. 4

~bserved although in some cases the activation energy was fairly small ( for trans

[

1+

Rh en 2 Cl 2

a value of 6 kcal was found).

[email protected]. Plot of E! vs. Spectral Shift of first Absorption Band relative 2

to the first Absorption Band of

[

Rh en 3

1

I

it

l

i q'~L...~

..... [~lev'~)lc I, ] '

l.l

o

%-

0.'1 041 e4

I

|

I

i

i

i

i

~

o.0

z,o

e,,o

6.O

vo

~o o

r# o

auo

a~ (-----~-)

The linear plot shown in Fig.l. indicates that as the energy of the process Rh (Ill) , (t2g6)

%,

Rh (1) , (t2g6 eg 2)

is decreased, by substitution of an en group by the hetero-ligands, the electroae reaction proceeds more readily. This energy is related to that required for the electronic rearrangement which must occur prior to the formation of the transition state at the electrode, whereby low-lying orbitals are vaoate~ for direct acceptance of electrons. The degree of electronic rearrangement is thus a direct function of the tetragonal splitting caused by ligands, X. It is of interest to note the order of reduction of the cis and trans forms of [ Rh en 2 CI 2]+. With the cobalt (III) anslogues it is found that the ~is isomer is reduced at more positive potentials than the trans form (3)- This has been explained by assuming preferential attraction of the

Vol. S, No. 4

DIACIDOTETRAMMINERHODIUM (111) COMPLEXES

293

ueg~tive end of the larger internal dipole in the c~s co~;~lex towards the mercury surface since it was found that waves appeared on the positive side of the electrocapillary

zero. Rhodium complexes, on the other hand, are

reduced at a negotive interface and the cis isomer is expected to experience greater repulsive effects than the tr2~s fform and to show reduction at more negative potentials than the t~c~s form as is observed.

References. 1. S.A.Johnson and ~.B~solo, Inorg.Chem., ~ 2. A.A.Vlcek, Discuss, Faraday Soc., ~

, 925 (1962)

164 (1958)

3. H.?.Holtzclaw and D.P. Sheetz, J.Amer.Chem.Soc., ~

3053 (1953)