Fa. Cotton et al., SINGLET-TRIPLET SEPARATIONS MEASURED BY P-31(H-1) NMR - APPLICATIONS TO QUADRUPLY BONDED DIMOLYBDENUM AND DITUNGSTEN COMPLEXES, Inorganic chemistry, 32(10), 1993, pp. 2104-2106
A series of quadruply bonded dimolybdenum and ditungsten compounds M2X
4(PP)2 (M = Mo, W; PP = bidentate phosphine ligands; X = Cl, Br, 1) wi
th internal rotational angles chi varying from 0.0 to 69.4-degrees hav
e been studied. Their P-31{H-1} NMR spectra are characterized by their
temperature-dependent shifts and line widths that broaden with increa
sing temperature. A nonlinear, least-squares fit of this temperature d
ependence of the paramagnetic shifts for their NMR signals allows the
evaluation of the singlet-triplet energy separation (-2J), the diamagn
etic shift (H(dia)), and the electron-nucleus hyperfine coupling const
ant (A). The singlet-triplet energy separations for all the compounds
investigated are found to be in the range 1200-3000 cm-1. It is now cl
early established that the ground state remains 1A1g (delta2) even at
chi = 45-degrees, where 3A2u (deltadelta) lies 1230 cm-1 above it. Th
e delta-bond energy and electronic delta-barrier can also be experimen
tally estimated as 13.8 +/- 0.5 kcal mol-1 and 10.3 +/- 0.5 kcal mol-1
, respectively.