Planar metal complex systems

Electrical Property Studies of Planar Metal Complex Systems... 

Electrical conductivity and thermopower measurements on single crystals of t(NHs) tCk (MGS) and (TTF) NiS C H (TTF = tetrathiafulvalene) illustrate the application of electrical property measurements to the study of intermolecular interactions and electronic structure in planar metal complex systems. MGS is an anisotropic, p-type semiconductor conductivity along the metal-chain direction ohm ... 

Fig. 1.—A molecular orbital coordinate system for a square planar metal complex.

Fig. 3.—Molecular orbital energy level scheme for square planar metal complexes in which the ligands themselves have a r-orbltal system (case 2).

In addition to systems with one-dimensional metallic properties, there are a considerably larger number of planar metal complexes where stacking interactions in the solid state give rise to unusual properties including highly anisotropic conductivity behavior but where electronic or structural factors lead to thermally activated conductivities 10, 11), Such one-dimensional semiconductors constitute an important area of study within the general topic of solids with one-dimensional interactions, and their study has provided much useful information regarding structure-property relationships. 

There are five basic forms for the trans or planar metal complexes which contain 1,4,8,11-tetraazacyclotetradecane, cyclam, or the hexamethyl analog, CTH. These forms arise due to the combinations of the configurations of the four asymmetric coordinated secondary amine nitrogens present in these systems. Each secondary amine proton is approximately in an axial position and either resides above or below the plane containing the nickel and four nitrogen atoms. Three of these forms are nonenantiomeric (I, II, and III, Fig. 1) and two are enantiomeric (IV and V, Fig. 1). The numerals 3 and 2 in Fig. 1 represent the three and two carbon chains, respectively, which span adjacent coordinated secondary amines. The + and — notation represents secondary amine protons above and below the plane,... 

Detailed ligand-field treatments of square-planar metal complexes have been provided by Martin and co-workers and Gray and co-work-ers,< 2,93) particular. Most of the discussion has centered on d complexes of halides and cyanides but other systems are reviewed by Gray.< The situation is not as simple as for cubic systems because more than one ordering of the subshells is feasible. Until recently both the energy level schemes b gix — y ) > bog xy) > a giz ) > Cgixz, yzy - and big(x — y ) > 2g( y) > y ) > Were serfously entertained for... 

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. 

The unfused cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and NMR spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. ... 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

In the early stage of the development of molecular conductors based on metal complexes, partially oxidized tetracyanoplatinate salts (for example, KCP K2 [Pt(CN)4]Br0.30-3H2O) and related materials were intensively studied [6], In this system, the square-planar platinum complexes are stacked to form a linear Pt-atom chain. The conduction band originates from the overlap of 5dz2 orbitals of the central platinum atom and exhibits the one-dimensional character. 

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