SYNTHESIS, STRUCTURE AND REACTIONS OF TRANS,TRANS-CR(H)(CO)2(NO)(PPH3)2. PREPARATION OF A STABLE PHENYLDIAZENE COMPLEX OF CHROMIUM(0)

Reaction of trans,trans-Cr(Br)(CO)2(NO)(PPh3)2 (1) with LiB(CH2CH3)3H in the presence of PPh3 gives trans,trans-Cr(H)(CO)2(NO)(PPh3)2 (2), w hich has been characterized spectroscopically and by single-crystal X- ray diffraction. The molecular structure of 2 exhibits a marked distor tion from octahedral geometry in that the CO and PPh3 ligands are bent toward the hydride site [angle C-Cr-C = 153.7 (2)-degrees; angle P-Cr -P = 165.5 (1)-degrees], as expected on steric grounds. Phenyldiazoniu m hexafluorophosphate reacts with 2, undergoing a 1,1-insertion into t he Cr-H bond to give [trans,trans-Cr(NH=NPh) (CO)2 (NO)(PPh3)2+][PF6-] (3). The phenyldiazene ligand of 3 undergoes nucleophilic displacemen t by bromide ion at 25-degrees-C to give 1. This contrasts with the mo lybdenum and tungsten homologues, which react with Br- at lower temper atures (-40 and 0-degrees-C, respectively) to afford the corresponding bromo complexes and free NH=NPh. Triflic acid reacts with 2 to afford the triflato derivative trans,trans-Cr(eta1-OSO2CF3)(CO)2(NO)(PPh3)2 (4), but unlike the molybdenum and tungsten derivatives, this complex did not prove to be a useful precursor to hydrazine and diazene comple xes of chromium.