Oxidative addition of N-substituted amides to molybdenum(II) involving nitrogen-hydrogen bond cleavage to give (.eta.2-N-acylamido-N,O)hydridomolybdenum(II) complexes: x-ray molecular structure of the seven-coordinate complex [MoH{N(COCH3)CH3}{(C6H5)2PCH2CH2P(C6H5)2}2]