Chemistry Reference
In-Depth Information
recorded between 300 and 4.2 K. The red quadrupole doublet refers to the HS-Fe II
ions, the one shown in blue to the LS-Fe II ions. The approximately equal reso-
nance areas of both doublets at 4.2 K indicates that the ST is incomplete and
apparently has occurred only at two of the four Fe II ions of a [2 9 2] grid unit. The
question arises whether the two Fe II ions exhibiting SCO are direct neighbors or
placed diagonal in opposite positions of the grid. In relation to what one has
learned from the studies of ST in dinuclear and trinuclear compounds (see Sects.
2.3.2.3 and 2.3.2.4 ), viz. that ST can cause a molecular distortion of a neighboring
complex with accompanying weakening of the ligand field, it is not unreasonable
to conclude that a similar process occurs in the present grid system. Thus it is
likely that the two complex molecules which have converted to the LS state at low
temperature are placed in diagonal positions as sketched in Fig. 2.26 [ 45 ].
2.3.2.6 Spin Transition in a 'Pentanuclear' Fe II
Coordination Compound
An unique pentanuclear Fe II compound with the formula [Fe 2 L 5 (NCS) 4 ] 2 [-
FeL 2 (NCS) 2 (H 2 O) 2 ] H 2 O, where L = 4-(p-tolyl)-1,2,4-triazole, has been synthe-
sized and structurally characterized [ 46 ]. The structure was found to consist of two
types of iron-containing structural units, a mononuclear unit and a dinuclear one
(Fig. 2.27 ). The mononuclear unit has a crystallographic inversion center, and is
coordinated by two NCS anions, two triazole nitrogen atoms, and two water mol-
ecules, each hydrogen-bonded to one of the two dinuclear units. The dinuclear units
Fig. 2.27 Molar fraction of HS molecules of the pentanuclear Fe II compound of formula
[Fe 2 L 5 (NCS) 4 ] 2 [FeL 2 (NCS) 2 (H 2 O) 2 ] H 2 O, derived from magnetic susceptibility measurements. A
sharp ST between HS and LS state of Fe II occurs near 111 K without hysteresis [ 46 ]. (Insert)
Structure of the pentanuclear Fe II compound [Fe 2 L 5 (NCS) 4 ] 2 [FeL 2 (NCS) 2 (H 2 O) 2 ] H 2 O. Thermal
ST takes place at the four outer Fe II ions with FeN 6 core, but not at the central Fe II ion with FeN 4 O 2
core, where the ligand field strength is too weak such that the Fe II ion remains in the HS state
Search WWH ::




Custom Search