Influence of the nephelauxetic effect on the kinetic and thermodynamic stability of cobalt (III) complexes of the tetramine type

by Tong, Ha-wai

Trans-[Co(cyclam)AX]n+, where A = N3-, X = CI-

A = NCS-, N3- and CN-, X = Br~ and A = NCS-, N3-, N02-

and CN-, X = H20, and the type trans-[Co(1~3)4AX]n+,where

A = N02-,N3-, NCS- and CN-, X = Cl-~ and A = N02-,CN-,

X = Br-, have been prepared. With the exception of aquo complexes, trans-[Co(cyclam)NCSBr]+ and trans-[Co(CYClam)N3Br]+ the ~netics of aquation and base hydrolysis of these complexes together with the base hydrolysis of trans-[Co(cyclam)NCSCl]+ and trans-[Co(cyclam)CNCl]+ have been studied over a range

of temperature. It was found, that, for a given pair of orienti~g

ligand A and leaving group X, the aquation and base hydrolysis rate constants, represented by kH and kOH respectively, of complexes of the type trans-[Co(am)4AX]n+ vary with the

nature of (am)4 in the following manner.

~ cyclam < (en)2 < (}lli3)4

kOH cyclam> (en)2 > (lt~3)4.

Furthermore, the r~lative stability of trans-[co(am)4ACl]+ with respect .to trans-[co(am)4AOH2]2+ for a given orienting ligand A, decreases in the order of (a~)4: cyclam> (en)2

> (lfH3)4.

The pKats of the trans-[Co(cYClam)AOH2]2+ complexes prepared have been measured at 25?. ~~en they

were compared with those analogous bisethylenediamine and

tetra-asmine complexes, it was found that, for a given

orienting ligand A, the acidity of these aquo conplexes

decreases in the order of cyclam > (en)2 > (lfH3)4?

The amine proton exch.ange kinetics of complexes

of the type trans-[co(am)4(NH3)2J3+ and trans-[co(am)4NH3Cl]2+ where (am)4 = cyclam, (en)2 and (NH3)4 have been studied at 34.50C. It was found that within the same molecule; the axial

ammonia molecules always exchange faster than the in-plane

cyclam and ethylenediamine molecules. As far as the variation of the exchange rate constants with the nature of (am)4 is concerned, it is similar to that of the base hydrolysis rate

constants. This results are consis~ent with an SN1cb mechanism.

All these results strongly support Poonfs' proposition concerning the influence of nephelauxetic effect on the thermodynamic and kinetic stability of these octahedral cObalt(II)?

amine complexes.

Reference.

,. C.K.Poon, J.Amer.Chem.Soc.,1970, 92, 4467.