Note Two typical cases of magnetism for dinuclear high-spin cobalt(II) complexes in trigonal-bipyramidal fields Md. Jamil Hossain, Hiroshi Sakiyama * Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Kojirakawa, Yamagata 990-8560, Japan Received 30 January 2002; accepted 24 April 2002 Abstract Previously, an acyclic ligand 1,3-bis[(2-dimethylaminoethyl)iminomethyl]benzene (baib) was found to form some dinuclear metal complexes [metal /manganese(II), cobalt(II), and zinc(II)], and crystal structures of the two cobalt(II) complexes [Co 2 (baib)(Me- CO 2 ) 3 ]BPh 4 (1) and [Co 2 (baib)(PhCO 2 ) 3 ]BPh 4 (2) were determined. Magnetic measurement was also carried out, but the magnetic analysis was not made. In this study, the cryomagnetic data for 1 were analyzed well by using isotropic Heisenberg model with the parameters g /2.30, J / /13.3 cm 1 , and Na /70 /10 6 cm 1 , indicating that a ground term does not possess an orbital angular momentum. On the other hand, however, the data for 2 could be interpreted using the so-called Lines’ theory with the parameters l / /175 cm 1 , k /0.63 and J / /1.0 cm 1 , indicating that a ground term possesses an orbital angular momentum. This difference was explained by the fact that the complex cation [Co 2 (baib)(PhCO 2 ) 3 ]  in 2 was much distorted than [Co 2 (baib)(MeCO 2 ) 3 ]  in 1, as found by the X-ray analysis. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Dinuclear cobalt(II) complexes; Magnetism; Theoretical method 1. Introduction Dinuclear cobalt complexes have attracted our inter- est since the active site structure of methionine amino- peptidase (MAP, EC 3.4.11.18) was revealed to contain a bis(m-carboxylato)dicobalt(II) core [1]. From the point of view of coordination chemistry, it is interesting and invaluable to study the relationship between dinuclear structures and their properties and functions. We are especially interested in the magnetism of dinuclear high- spin cobalt(II) complexes. This is a challenging area because the magnetic analysis of dicobalt(II) complexes is usually accompanied by difficulties due to the orbital angular momentum. In general, the orbital angular momentum is partially quenched in a ligand field of a certain symmetry [2]. For instance, in a ligand field of O h symmetry, only the orbital angular momenta of T terms (T 1g ,T 1u ,T 2g , and T 2u ) remain as a result of the quenching. When the symmetry is decreased to D 3h , only E terms (E? and Eƒ) have orbital momenta. Thus, the effect of the orbital momentum is highly dependent on the symmetry around the metal ion. In the magnetic analysis of dinuclear high-spin cobalt(II) complexes, the spin-only Heisenberg model H / /JS × / S is not suitable for magnetic analysis when the ground term has a first-order orbital momentum. Instead of this treatment, Lines developed a theory considering the spin-orbit coupling for isotropic pure octahedral geometry [3], and recently Sakiyama devel- oped a new theoretical method for axially distorted octahedral geometry [4]. Since the magnetic behavior is strongly affected by the orbital momentum of a ground term determined by the symmetry, our next target is to study the magnetism of another symmetry, such as that of the D 3h point group. In the case of trigonal-bipyramidal geometry, the symmetry belongs to the D 3h point group. The lowest orbital state of the free cobalt(II) ion is 4 F; the second lowest is 4 P; and the third is 2 G. In the weak field of D 3h symmetry [5], the 4 F term splits into the 4 A 2 ? , 4 A 1 ƒ , 4 A 2 ƒ , 4 Eƒ, and 4 E? terms, in that order, and the 4 P term into * Corresponding author. Tel.:  /81-23-628-4601; fax:  /81-23-628- 4591 E-mail address: saki@sci.kj.yamagata-u.ac.jp (H. Sakiyama). Inorganica Chimica Acta 338 (2002) 255  /259 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)01053-8