SINGLET-TRIPLET SEPARATIONS MEASURED BY P-31(H-1) NMR - APPLICATIONS TO QUADRUPLY BONDED DIMOLYBDENUM AND DITUNGSTEN COMPLEXES

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.