Substituent effect of the coordinated pyridine in a series of pyrazolato bridged dinuclear diiron(II) complexes on the spin-crossover behavior

Two series of pyrazolato bridged dinuclear NCS and NCBH3 diiron(II) complexes with various types of 3- or 4-substituted pyridines, [{Fe(NCS or NCBH3) (X-py)}(2)(mu-bpypz)(2)], were prepared and their variable-temperature magnetic susceptibilities were measured. There were found linear correlations of the spin-crossover temperatures T-c not only between the NCS and NCBH3 complexes with the corresponding substituted pyridines, but also between the T-c and the Hammett constants, supporting the electronic substituent effect of the coordinated pyridine rather than a steric effect. The ligand field and the interelectronic repulsion parameters together with the thermodynamic data and/or cooperativity factor were discussed in relation with their spin-crossover behavior.