The complex anion [Fe(CN)
5
(NO)]
2
(nitroprusside) has been de
fi
ned intact using electrospray ionization mass
spectrometry (ESIMS), with minor daughter ions predictable fragments of the parent [Fe(CN)
5
NO]
2
. Direct
evidence for ion pairing of [Fe(CN)
5
NO]
2
with monovalent alkali metal and some alkylammonium cations was
found independently using ESIMS and
13
C NMR techniques. The results are relevant to the signi
fi
cant in
fl
uence of
the concentration and nature of cations on the solution equilibrium between nitroprusside with thiolates and their
nitrosothiol adducts that presumably arises through charge reduction in tight ion pairs facilitating adduct formation.
Ion-paired species {M
[Fe(CN)
5
NO]
2
}
, {M
3
[Fe(CN)
5
NO]}
and more complex clusters of the type {M
x
[Fe(CN)
5
-
NO]
y
}
z
/
z
(up to
x
=
8 and
y
=
5) were observed by ESIMS. The trend in the ion pair formation seen by ESIMS
follows the series Li
> Na
> K
> Rb
> Cs
and Me
4
N
> Et
4
N
> Pr
4
N
> Bu
4
N
and is consistent with purely
electrostatic expectations for unsolvated ions. Whereas the poorly solvated alkylammonium series bulk solution
behaviour resembles that found by the ESI technique, the activity of alkali metal cations proceeds conversely,
i.e
. Li
< Na
< K
< Rb
< Cs
, veri
fi
ed by a
13
C NMR study, because the hydrated cation radii play a crucial role in the
bulk solution in those cases. Di
ff
erences in changes of the chemical shift upon addition of various cations points to
the axial CN
position as especially important for binding to these cations. Further, short-lived adducts with thiolates
of the type {M
3
[Fe(CN)
5
NO(C
4
H
4
O
4
S)]
4
}
have been de
fi
ned by their observation in ESIMS directly and through
their decomposition products, providing further support for reversible nitrosothiol complex formation. Mercapto-
succinate ion pairing with cations, which would make formation of the proposed nitrosothiol complex easier, is also
observed by ESIMS. The dithiolato complex {K
[Fe(CN)
5
NO(C
4
H
6
O
4
S)
2
]
2
}
, as well as the disul
fi
de anion radical
were detected; both species are suggested intermediates in the spontaneous and autocatalyzed redox decomposition
of the [Fe(CN)
5
N(O)SR]
(
n
2)
complexes. The ESIMS study has thus provided information relevant to both
thermodynamic and kinetic processes in a reaction where species of limited stability are involved.