Triplet state transition metal complexes

Not classified as free radicals are atoms or molecules in ground or excited electronic states wifli multiplicities larger than two (e.g. O, P O2, X N, " 8 molecules in excited triplet states), transition metal ions and their complexes deriving their paramagnetism exclusively or mainly from d- and f-eleclrons and charge transfer complexes. However, a number of polyatomic molecular species which do not fidfill die above definition are included because their properties closely resemble those of stmcturally closely related free radicals. These are... 

Electrophilic and nucleophilic phosphinidene complexes have been related to the corresponding carbene complexes of which the Fischer-type is usually considered as a singlet-singlet combination and the Schrock-type as a triplet-triplet combination. However, both the strongly preferred triplet state of R-P and the M=P bond analysis suggest this schematic interpretation to be less appropriate for transition metal complexed phosphinidenes. 

An increase in the ion annihilation exergonicity AG to values comparable to the excited triplet-state energies (AG I LT < 0) opens an additional electron transfer channel (T-route). In the simplest case, only one excited triplet 3 A or 3 D becomes accessible. Triplet emission can be directly observed from the ECL systems involving rare earth and transition metal complexes with allowed (due to extensive spin-orbit coupling) triplet-singlet electronic transition. 

The use of transition or rare earth metal complexes as emitting materials for OLED has attracted much attention recently because of the enhancement in EL efficiency from triplet excited state.16 Chan and co-workers demonstrated that incorporation of ruthenium bipyridyl complexes into conjugated polymers would enhance the charge carrier mobilities.17 Through attachment of different transition metal complexes onto polymer chains, tuning of light-emitting properties could be achieved.18... 

Photosensitized oxidations involve either organic dyes or transition metal complexes as sensitizers. Most organic sensitizers operate in their triplet states, as they are relatively longer lived than the singlet states. 

For those familiar with NMR spectroscopy it may be helpful to realize that the ESR g-shift is comparable with the NMR chemical shift. Similarly, electron-nuclear hyperfine coupling can be compared with nuclear-nuclear spin-spin coupling in NMR. (In systems containing more than one unpaired electron per molecule, electron spin-electron spin coupling is, of course, important. For doublet-state radicals, this coupling does not arise it is of great importance in triplet state molecules and in many high-spin transition metal complexes.)... 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Cr(C6Hii)4 is tetrahedral and a triplet however, square-planar discriminating factor. Another factor at work in these molecules is the spin pairing energies. As indicated previously, 3d valence orbitals are considerably more contracted that 4d and 5d thus the Coulomb repulsion integrals are much larger for first-row transition metal complexes and high-spin states are more favored than is the case for second and third row complexes. 

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