Molecular squares, rectangles, and corners: Ground-state electronic structure and configurational properties

A recently developed class of molecular squares based on octahedral ReCl(CO) ₃R ₂ cornerposts has until now lacked a detailed theoretical description. We present an analysis of electronic structure of five of the simplest squares, rectangles, and related fragments; that is, those with pyrazine and bipyridine walls. We further consider a square containing Zn - porphyrin walls. The derived electronic charge distributions reveal bonding structure and spectral features of the class. We show that the analysis of conformational preferences can be greatly simplified by using a "piecewise" strategy, which enables us to make several generic statements about conformations of this class of macromolecules. To further illustrate advantages of this approach, we find the origin of metastability of the planar conformation of the bipyridine square.