The four-co-ordinate nickel(
II) macrocyclic complex
(R,S,R,S)-[NiL][ClO
4
]
2
, where L is
1,3,6,8,12,15-hexaazatricyclo[13.3.1.1

8,12
]icosane, has been isolated. It shows a significantly weaker ligand
field with an increased absorption coefficient and more positive
electrochemical oxidation and reduction potentials compared with those
of the R,R,S,S isomer. Its crystal
structure was determined by X-ray crystallography. It shows a
square-planar geometry with a large tetrahedral distortion of the four
nitrogen donors (maximum ± 0.135 Å). The
average Ni–N bond distance is 1.938(3) Å for the
secondary nitrogens and 1.969(3) Å for the tertiary. The
complex isomerizes to the R,R,S,S
complex at 50 °C in an aqueous solution containing 9.1% (v/v)
MeCN. The first-order rate constant k
obs
is
4.16 × 10
-5
s
-1
at pH 7 and I = 1.0 mol
dm
-3
(LiClO
4
). The value of
k
obs
is independent of pH and buffer concentration,
but increases with increase in water content and decrease in ionic
strength. The activation parameters are
ΔH
‡
= 33
± 1 kcal mol
-1
and
ΔS
‡
= 24 cal
K
-1
mol
-1
at 25 °C. It
is proposed that isomerization of the
R,S,R,S to the
R,R,S,S complex proceeds
via axial co-ordination of water followed by a
Ni–N(tertiary) bond dissociation assisted by a water molecule in
the second co-ordination sphere acting as a general acid.