Mixed-valence P,P-bonded diphosphorus compounds, doubly bridged by N,N '-dimethylurea: IV. An ab initio characterization of P-P bonds in model diphosphorus compounds

We have carried out ab initio electronic structure calculations at the HF/6 -31G* level using the GAUSSIAN 94 program for a number of diphosphorus comp ounds in which the two directly bonded phosphorus atoms are bridged by one or two urea groups. These molecules serve as structural models for a large number of dialkylurea bridged diphosphorus compounds which have been synthe sized and reported in the literature. Comparisons are made between the stru ctures of our model compounds and the observed structures of the dialkyl ur ea compounds. For calibration purposes comparisons are also made between ca lculated and observed structures of simpler diphosphorus compounds. We find that the urea bridging groups have only a small effect on the P-P equilibr ium separations and their sensitivity to protonation and other additions. T hus we conclude that the observed insensitivity of the P-P separations to q uaternization, oxidation, or reduction, is not so much a consequence of con straints due to intramolecular bridging by dialkylurea groups as it is an i nherent property of the diphosphorus compounds themselves. We also conclude that the introduction of bridging urea groups into diphosphorus compounds has as its primary structural consequence the stabilization of eclipsed con formations. (C) 1999 Elsevier Science B.V. All rights reserved.