Excited states square-planar bonding

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

Molecules in excited states can also relax along nontotally symmetric normal coordinates when their potential surfaces have energy minima along one of those coordinates. This type of distortion is rarer than the totally symmetric distortions and the theoretical treatment is more complicated. Only two simple examples are discussed in this chapter. The one most easily visualized is the distortion in square planar PtCl in which two of the Pt—Cl bonds opposite to each other elongate and the other two contract. Distortion along this nontotally symmetric stretching mode in an excited electronic state results in a rhombus (Dj ). 

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