Abstract
Manganese(III) forms both bismalonatodiaquo- and trismalonato-complex ions. Several new malonatomanganese(III) complexes have been prepared of the types M[1][(C[3]H[2]O[4])[2](H[2]O)[2]Mn]xH[2]O and M[1][(C[3]H[2]O[4])[2]Mn]. The established preparative methods have been improved for the bis- and tris-series and a new route to the bisoalonatodiaquo-complexes, which involves the oxidation of manganese(II) by permanganate, has proved successful. The reflectance spectra of the complexes have been measured although these could not be interpreted in an unequivocal manner. Magnetic susceptibility measurements, over a range of temperature, confirm that the complexes contain the high-spin d4 manganese(III) cation with moments close to the spin-only value. The infrared spectra of the complexes are similar and cannot be used to distinguish the bismalonatodiaquo- and trismalonato-canganese(III) ions. Citratomanganese(III) complexes cannot be prepared by routine preparative methods. Solution studies using a potentiometric method, have shown that manganese(III) forms both mono- and biscitrato complexes in solution with logbeta[1] = 18.51, and logbeta[2] = 21.78. The stability constants for citratoman-ganese(II) complexes have been measured and it was found that logbeta[1] = 3.014 and logbeta[2] = 4.97. Evidence is also presented to show that metal complexes of the monoprotonated citrate ion are formed in acid solutions of manganese(II) with logbeta[1] = 2.19 and logbeta[2] = 4.096. Stability constants in the malonatomanganese(III) system have also been measured and it was found that logbeta[2] = 9.41 and logbeta[3] = 10.65. Using the stability constants measured in the present work and some literature values, the stability of manganese(III) complexes is related both to the ring size formed by a chelating group and to the number of chelate rings formed by any one ligand group.