Spin “cross-over

D. Low-spin Iron(II) Complexes 1. Spin Cross-over... 

Spin cross-over kinetics for Fe(II) complexes 26,103) have been reviewed. The l.s. h.s. (1A1 5T2) transition in solution is character-... 

The synthesis of the N03 and PFg salts of the iron(III) complex of the terdentate phenolate anion of the liquid crystal Schiff base (7) provides the first example of the coexistence of spin transition and liquid crystal properties in a single compound. The ligand now needs to be tuned to increase the spin cross-over transition temperature and decrease the transition temperature to the liquid crystal state. ... 

A different, indeed unique, type of spin cross-over behavior is claimed for the mixed valence dithiooxalate salt Pr 4N[Fe Fe (dto)3]. Here the spin change is associated with electron transfer, with the bridging dithiooxalate favoring Fe —S bonding at higher temperatures, Fe —O bonding at lower temperatures ... 

The frequent occurrence of thiocyanate in spin cross-over complexes is documented, in Section 5.4.1.4 above and in connection with diimine and with triazole ligands in Sections 5.4.3.5.9 and 5.4.3.6.3 below, while complex formation with iron(III) appears in Section 5.4.5.1.4. 

Aminomethylpyridine (picolylamine) is an important ligand in respect to spin cross-over, [Fe(2-pic)3]Cl2 being the key compound." Fleat capacity measurements on [Fe(2-pic)3]Cl2 EtOH gave values of 6.14kJmol and 50.59 JK moC for the spin eross-over entropy the determined entropy was analyzed into a spin contribution of 13.38, an ethanol orientational effeet of 8.97, and a vibrational contribution of 28.24 JK mol. " This compound exhibits weak cooperativity in the solid state." The heat capacity of [Fe(2-pic)3]Cl2 MeOH is consistent with very weak cooperativity." [Fe(2-pic)3]Br2 EtOH shows a lattice expansion significantly different from that expected in comparison with earlier-established behavior of [Fe(2-pic)3]Cl2 EtOH." ... 

The iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

The structures of the bis-ligand iron(II) complexes of the tripodal ligands tris(pyrazol-l-yl)methane and bis(pyrazol-l-yl)(pyridin-2-yl)methane have been determined the former is a spin cross-over complex." " Polybis(pyrazolateiiron(II) has a ID chain structure with the Fe " " ions doubly bridged by pyrazolates the Fe + ions (-S = 2) are weakly antiferromagnetically coupled. ... 

Spin cross-over iron(II)-triazole and -tetrazole complexes have been reviewed. 

The cyclic Schiff base terimine ligand (161) forms a spin cross-over ternary iron(II) complex [Fe(161)(CN)2]. This has intermediate magnetic behavior, indicating a 50 50 mixture of high-spin... 

Spin cross-over behavior has been established for a number of Schiff base complexes of iron(III) complexes with N402-donor sets. A two-step spin cross-over in the warming direction for [Fe A-(8-quinolyl)salicylaldiminate 2](NCSe) has been ascribed to intermolecular TT-interactions between aromatic rings. Usually spin cross-over in these N402-complexes is... 

The preceding discussion should indicate how the chemically interesting phenomenon of spin cross-over becomes quite a complex one to understand in the solid state. It must be pointed out that in spite of the extensive studies, there are many aspects of spin-state transitions (e.g. critical behaviour, soft modes) that are yet to be explored. 

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