SOLVATION STRUCTURES AND KINETICS OF SOLVENT EXCHANGE-REACTIONS OF THE MANGANESE(II) ION IN 6 NITRILES AS STUDIED BY X-RAY-DIFFRACTION, EXAFS, AND NMR TECHNIQUES

The solution structure of the manganese(II) ion in acetonitrile (methy l cyanide, AN) has been determined by an X-ray diffraction technique: there are 6.0 +/- 0.2 Mn-N bonds and the Mn-N bond length is 221 +/- 1 pm. The solvation structures of the manganese(II) ion in propiononitr ile (ethyl cyanide, PN), butyronitrile (propyl cyanide, BuN). isobutyr onitrile (isopropyl cyanide, (BuN)-Bu-i), valeronitrile (butyl cyanide , VN), and benzonitnile (phenyl cyanide, BzN) have been determined by extended X-ray absorption fine structure spectroscopy. The manganese(I I) ion in all of these nitriles is six-coordinate octahedral with an M n-N bond length of 221 +/- 1 pm. The activation parameters for solvent exchange on the manganese(II) ion in the six nitriles have been deter mined by the nuclear magnetic resonance line broadening technique at v arious temperatures and pressures. The values of the rate constant (k( ex)(298), s(-1)) activation enthalpy (Delta H-double dagger, kJ mol(-1 )), activation entropy (Delta S-double dagger, J mol(-1) K-1), and act ivation volume (Delta V-double dagger, cm(3) mol(-1)) for the nitrile exchange are 1.3 x 10(7), 28.6 +/- 0.5, -13 +/- 2, and -5.8 +/- 0.2 in AN; 1.3 x 10(7), 29.6 +/- 0.7, -10 +/- 3, and -2.1 +/- 0.4 in PN; 9.9 x 10(6), 31.3 +/- 0.8, -6 +/- 3, and -5.0 +/- 0.8 in BuN; 1.1 x 10(7) , 40.0 +/- 0.4, 24 +/- 1, and -2.5 +/- 0.7 in (BuN)-Bu-i; 9.3 x 10(6), 35.6 +/- 0.7, 8 +/- 3, and +0.6 +/- 0.6 in VN; 1.2 x 10(7), 36.9 +/- 0.8. 14 +/- 3. and none in BzN, respectively. The values of Delta H-do uble dagger and Delta S-double dagger gradually increase with increasi ng bulkiness of the nitrile molecule, and the reaction mechanism is co ncluded to be less associative with increasing the bulkiness, because the Mn-N bond energy in the nitrile-solvated manganese(II) ion is simi lar in all nitriles. The change in the Delta V-double dagger value is discussed in terms of the change in the size of the inner sphere.