Non-Berry pseudo-rotation

Two papers looking at the photochemistry of iron carbonyls touching upon aspects of their photochemistry that are central to the work below are discussed now. The paper of Hubbard and Lichtenberger [36] from 1981 examined the photoelectron spectrum of Fe(CO)5 in the gas phase. This paper is of relevance as they claimed to have evidence of Jahn-Teller distortions in the Fe(CO)5 cation. Here for the first time it is explicitly mentioned that highly symmetrical transition metal complexes in general have good potential for observable Jahn-Teller activity with regards to their photochemistry after ionization and/or dissociation. They found that ionization into the E state showed Jahn-Teller activity and discussed this in terms of non-Berry pseudo-rotation. 

Fig. 11 Representation of lowest adiabatic potential of singlet (S = 0) and triplet (S = 1) Fe(CO)4 around T Jahn-Teller conical intersection at tetrahedral (7 ) geometry. There are three equivalent two-dimensional troughs in the space spanned by each pair-wise selection of equal L-M-L angles (boxed vs unboxed). The topological connectivity where the troughs intersect is indicated. There are two non-equivalent epikemel distortion directions E[ 2(Td,h) leading to 6 equivalent C2v minima ( ), and 12 equivalent Cs(x) saddle-points respectively. The non-Berry pseudo-rotation barrier is very small ( 5kcal mol ). CASSCF optimised geometrical parameters for singlet and triplet states are shown at the top left...

Fig. 12 Octahedral representation of Fe(CO)4 T (8) t Jahn-Teller surface (originally devised by Poliakoff and Ceulemans in [66]). The non-Berry pseudo-rotation paths shown in Fig. 11 are the paths between opposite vertexes on the octahedron, which are C2v minima ( ) reached by following the forward and reverse directions of epikernel E. The Cj saddle-points (x) lie at the centre of each edge...

Single-crystal X-ray diffraction (at 109 K) data show that PF5 has a trigonal bipyramidal structure, 15.32. In solution, the molecule is fluxional on the NMR spectroscopic time-scale and one doublet is observed in the NMR spectrum, i.e. all F environments are equivalent and couple with the P nucleus. This stereochemical non-rigidity is another example of Berry pseudo-rotation (see Fig. 4.24). 

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