ORGANOTRANSITION-METAL COMPLEXES OF MULTIDENTATE LIGANDS .16. ON THE NATURE OF THE SIGMA-DONICITY OF THE SATURATED NITROGEN LIGANDS - CHELATE-ASSISTED WEAKENING OF THE ALPHA-N-H BOND - SYNTHESIS, AND SPECTRAL AND STRUCTURAL STUDY OF [MO(N-N)(CO)4] (N-N = SATURATED NITROGEN BIDENTATE LIGANDS)

A series of complexes, [Mo(N-N)(CO)4] (N-N = saturated nitrogen bidentate ligands), were prepared from [Mo(pip)2(CO)4] (pip = piperidine) and N-N in CH2Cl2 at 40-degrees-C. The structure of [Mo{(PhCH2)HNCH2CH2NMe2}(CO)4] was also determined by X-ray crystallography: space group Pbca, a = 14.895(7), b = 11.947(4), c = 19.189(8) angstrom, V = 3415(2) angstrom 3, Z = 8, d(calc) = 1.503 g/cm3, R = 0.026 and R(w) = 0.026 based on 1646 reflections with I > 3.0 sigma(I). Both the spectral and structural data of the complexes indicate that the electron-density donating from the ligands comprises not only the lone electron pair of the nitrogen atom but the electron-density deprived from the neighboring groups by the highly electronegative nitrogen atom. These data also show that the more electron density of the alpha-N-H bond relative to that of the alpha-N-R bond (R = alkyl or aryl) is removed upon coordination of the nitrogen donor ligands, apparently explaining why a chelate-assisted oxidative addition of the N-H bond occurs more readily than the N-R bond through prior coordination and concurrent weakening of the N-H bond.