Square-planar complexes excited states

Almost all square planar complexes with simple ligands are diamagnetic and contain a metal ion with the -electronic configuration. Thus the ground state is lAig. The lowest energy excited states will be described separately for case 1 and case 2. 

Orbital and Interelbctronic-repulsion Energies of Some Excited States of Interest for an nda Square Planar Complex... 

In addition to photosubstitution and photoelimination reactions, in the cases of some Ni(II) complexes, photoexcitation of square-planar complexes Ni(TP) and formation of the photoassociative ligand-field (LF) excited state 3Blg can lead to photoaddition reactions yielding hexacoordinate complexes Ni(TP)L2 [65, 66, 75-77], Such processes differ from the second step of photosubstitutions since an excited complex participates in them and the addition is conditioned by the electronic structure of the complex in its excited state (see Table 3). 

Large geometrical distortion in the lowest excited states of Pt(gly)2 is also suggested by spectroscopic studies, which have indicated that similar square planar complexes have pseudotetra-hedral excited states (93a), by semi-empirical molecular orbital calculations (56), and also by recent quenching and sensitization studies of the cis trans reaction (Section lII-D-1). ... 

The photosubstitution studies of the Co, Rh, Ir triad are concerned with the hexacoordinated d -M(IIl) complexes however, photoreactions of the d -[IrClj] and a few d -M(I) square-planar complexes also are known. The d -amine complexes of Co(Ill), e.g., [Co(NH3) ] , are poor models for the photosubstitution reactions of other low-spin d complexes, which, like such Co(III) cyano ions as [Co(CN) ] "", have lowest-energy excited states (ES) that are ligand-field (LF) triplets with the (t p fe electronic configuration (Fig. 1). For the Co(III) amines, LF quintets with the (tjP (ep configuration are the lowest states. Regardless of the theoretical implications of such electronic differences, the LF photochemistry of the Co(III) amines contrasts with that of analogous Rh(III) and Ir(III) complexes and of the Co(III)-cyano complexes. For the latter systems in solution, irradiation into LF absorption bands leads to photosubstitution with moderate (ca. 0.1) quantum yields independent of the irradiation wavelength, A. ... 

If the distortions in the complex are weak, the energy order of the excited states will be Aijsquare-planar complex, the relative term energies E< are determined by the following criteria ... 

In addition to excitations into the id (or Ad) shells, XANES can also be used to probe higher-lying excited states. For atomic spectra, a complete series of Rydberg transitions can be seen. For first transition series metals, the allowed Is Ap transition is sometimes observed. This is the assignment given to the intense transition observed on the rising edge for Cu and for some square-planar Cir and Ni complexes. From studies of model compounds, it is found that the Is Ap" transition is intense for square planar complexes but weak for tetrahedral... 

The predominant photoreaction of the [Pt(C2H4)Cl3] anion is loss of ethylene, which is accompanied by a little loss of m-chloride. This behaviour contrasts with the corresponding thermal reaction, where the predominant reaction is loss of the chloride trans to the ethylene. This contrast between thermal and photochemical behaviour is novel most photochemical reactions of platinum(ii) complexes follow the same course as the corresponding thermal reactions, but occur more quickly. The photoaquation of the ethylene ligand is subject to sensitization by acetone or by acetophenone, and therefore probably occurs via the first excited singlet state. A general model for photosubstitution reactions has been described, which is relevant to octahedral as well as to square-planar complexes. ... 

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. 

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