Three air-stable Co(II) mononuclear
complexes with different aromatic substituents have been prepared
and structurally characterized by single-crystal X-ray diffraction.
The mononuclear complexes [Co(H<sub>2</sub>L1)<sub>2</sub>]·2THF
(<b>1</b>), [Co(HL2)<sub>2</sub>] (<b>2</b>), and [Co(H<sub>2</sub>L3)<sub>2</sub>]·CH<sub>2</sub>Cl<sub>2</sub> (<b>3</b>) (where H<sub>3</sub>L1, H<sub>2</sub>L2, and H<sub>3</sub>L3 represent 3-hydroxy-naphthalene-2-carboxylic acid (6-hydroxymethyl-pyridin-2-ylmethylene)
hydrazide, nicotinic acid (6-hydroxymethyl-pyridin-2-ylmethylene)
hydrazide, and 2-hydroxy-benzoic acid (6-hydroxymethyl-pyridin-2-ylmethylene)
hydrazide, respectively) feature a distorted <i>mer</i> octahedral
coordination geometry. Detailed magnetic studies of <b>1</b>–<b>3</b> have been conducted using direct and alternating
current magnetic susceptibility data. Field-induced slow magnetic
relaxation was observed for these three complexes. There are few examples
of such behavior in (distorted) octahedral coordination geometry (OC)
Co(II) mononuclear complexes with uniaxial anisotropy. Analysis of
the six-coordinate Co(II) mononuclear single-ion magnets (SIMs) in
the literature using the SHAPE program revealed that they all show
what is best described as distorted trigonal prismatic (TRP) coordination
geometry, and in general, these show negative <i>D</i> zero-field
splitting (ZFS) values. On the other hand, all the Co(II) mononuclear
complexes displaying what is best approximated as distorted octahedral
(OC) coordination geometry show positive <i>D</i> values.
In the new Co(II) mononuclear complexes we describe here, there is
an ambiguity, since the rigid tridentate ligands confer what is best
described for an octahedral complex as a <i>mer</i> coordination
geometry, but the actual shape of the first coordination sphere is
between octahedral and trigonal prismatic. The negative <i>D</i> values observed experimentally and supported by high-level electronic
structure calculations are thus in line with a trigonal prismatic
geometry. However, a consideration of the rhombicity as indicated
by the <i>E</i> value of the ZFS in conjunction with the
SHAPE analysis shows that in this case it is difficult to distinguish
between the OC and TRP descriptions
