Mixed-valent Mn/O dinuclear and polynuclear molecular compounds containing Mn
III
are almost without exception trapped valence. Large differences between the strengths of the exchange interactions within Mn
II
Mn
III
, Mn
III
Mn
III
and Mn
III
Mn
IV
pairs lead to situations where Mn
III
Mn
IV
interactions, the strongest of the three mentioned and antiferromagnetic in nature, dominate the intramolecular spin alignments in trinuclear and higher nuclearity mixed-valent complexes and often result in molecules that have large, and sometimes abnormally large, values of molecular spin (
S
). When coupled to a large molecular magnetoanisotropy of the easy-axis-type (negative zero-field splitting parameter,
D
), also primarily resulting from individual Jahn–Teller distorted Mn
III
centres, such molecules will function as single-molecule magnets (molecular nanomagnets). Dissection of the structures and exchange interactions within a variety of mixed-valent Mn
x
cluster molecules with metal nuclearities of Mn
4
, Mn
12
and Mn
25
allows a ready rationalization of the observed
S
,
D
and overall magnetic properties in terms of competing antiferromagnetic exchange interactions within triangular subunits, resulting spin alignments and relative orientation of Mn
III
JT axes. Such an understanding has provided a stepping stone to the identification of a ‘magnetically soft’ Mn
25
cluster whose groundstate spin
S
value can be significantly altered by relatively minor structural perturbations. Such ‘spin tweaking’ has allowed this cluster to be obtained in three different forms with three different groundstate
S
values.