CRYSTAL-STRUCTURES, VIBRATIONAL-SPECTRA A ND NORMAL-COORDINATE ANALYSES OF CIS-[OSX2(ACAC)(2)] AND TRANS-[OSX2(ACAC)(2)], X=CL, BR, I

The crystal structures of trans-[OsCl2(acac)(2)] (triclinic, space gro up P (1) over bar, a = 7.4114(5), b = 7.6419(9), c = 7.9944(6) Angstro m, alpha = 62.692(7), beta = 87.687(6), gamma = 60.667(6)degrees, Z = 1), trans-[OsBr2(acac)(2)] (triclinic, space group P (1) over bar, a = 7.467(3), b = 7.621(3), c = 8.260(3) Angstrom, alpha = 115.03(3), bet a = 92.78(3), gamma = 117.91(3)degrees, Z = 1), cis-[OsCl2(acac)(2)] ( monoclinic, space group C2/c, a = 13.8532(13), b = 7.7990(8), c = 13.6 202(12) Angstrom, beta = 108.375(10)degrees, Z = 4) and cis-[OsBr2(aca c)(2)] (monoclinic, space group C2/c, a = 13.944(2), b = 8.0347(13), c = 13.743(2) Angstrom, beta = 106.757(12)degrees, Z = 4) have been det ermined by single crystal X-ray diffraction analysis. To enhance the s pectroscopic resolution, the IR and Raman spectra of the six complexes have been measured at low temperature (10 K). Using the molecular par ameters of the X-ray determinations normal coordinate analyses based o n a modified valence force field have been performed for trans-[OsCl2( acac)(2)], trans-[OsBr2(acac)(2)] and cis-[OsCl2(acac)(2)], and the no rmal modes of vibration assigned. With sets of 31 or 32 force constant s, taking into account the innerligand vibrations, a good agreement be tween observed and calculated frequencies has been achieved. The valen ce force constants for the X-Os-X axes are f(d)(OsCl) = 1.81, f(d)(OsB r) = 1.61 mdyn/Angstrom, and for the Cl'-Os-O-. axis are f(d)(OsCl') = 1.94, f(d)(OsO.) = 2.81 mdyn/Angstrom and f(d)(OsO) ranges from 3.27 to 3.31 mdyn/Angstrom.