N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

Cyclometallated complexes incorporating a heterocyclic donor atom; the interface of coordination chemistry and organometallic chemistry

Contrasting reactivity of fluoropyridines at palladium and platinum: C-F oxidative addition at palladium, P-C and C-F activation at platinum

Catalytic C-C coupling reactions at nickel by C-F activation of a pyrimidine in the presence of a C-Cl bond: The crucial role of highly reactive fluoro complexes

Flavoenzymes are characterized by their remarkable versatility and strict specificity. The former can be grasped when flavoenzymes are treated as a whole, while the latter refers to each flavoenzyme in which the broad versatility of flavin is specifically controlled. The versatility stems from the variety of the redox, ionic, and electronic states that the flavin ring system can adopt. Versatility of flavoenzymes is reflected in their classification, which has generally been based on substrates and reactions catalyzed. A different classification is presented according to the number of electrons transferred in the reductive and oxidative half reactions. Specificity of each flavoenzyme is understood in terms of the regulatory mechanism of the broad reactive potentiality of flavin. The elements of this regulatory mechanism include hydrogen-bonding network, electrostatic effect, charge-transfer interaction, positioning between a substrate/ligand and flavin, and modulation of resonance hybridization, each of which is explained with relevant examples provided mainly by studies from the author's group.

Versatility and specificity in flavoenzymes: control mechanisms of flavin reactivity.

trans-Bromo(2-, 3-, and 4-pyridyl)bis(triethylphosphine)palladium(II) complexes

A new parameterization for the first transition metal has been proposed in the framework of CNDO/2 method. We carried out CNDO/2 calculation of hexamine complexes [M(NH3)6]2+ and hexa-aquo complexes [M(OH2)6]2+ in the high spin state where M = Mn, Fe, Co, Ni, and Cu, using new parameters. It is shown that the calculated order of binding energy is MnL < FeL < CoL < NiL ≈ CuL (where L means the ligand), and is in good agreement with experiment. We discussed how the orbital nodes affect the nature of bonding between metal and ligand.

Electronic structure of transition‐metal complexes. I. Parametrization for CNDO/2 method

Hydrogen bonding of flavoprotein: II. Effect of hydrogen bonding at hetero atoms of reduced flavin on its reactivity

CNDO/2 calculations of pentaammine complexes, [M(NH3)5X] (both low and high spin states), where M=Co2+, Co3+, Fe2+, and Fe3+, and X=FH, OH2, CO, NH3, pyridine, OH−, F−, and CN−, have been carried out in order to investigate the mutual influence between ligands. The present calculation shows that the σ-type mutual influence is remarkably affected by the oxidation number and the spin state of the metal ion and scarecely affected by the number of d electron (d electron configuration). Furthermore, the calculated results show the metal-ligand bond distance of a complex in the low spin state should be shorter than that in the high spin state.

Electronic structure of transition metal complexes. II. Trans influence in ammine complexes of the first transition metals.

In order to elucidate π-type interaction between the metal and the ligand, the electronic structure of various metal porphyrins, [M(Por)], where M=Mn, Fe, Co, Ni, and Cu, has been investigated by the CNDO/2 method. The present calculation shows that the nature of σ-type interaction in M–N bond of porphyrin complex is similar to that of the corresponding ammine complex. The calculated ionization potentials reproduce the experimental data. The electron population of macrocycle π-electron systems in metal porphyrins is about 18 which accords with Huckel’s 4n+2 rule.

Eiko Kai

Papers

trans-Bromo(2-, 3-, and 4-pyridyl)bis(triethylphosphine)palladium(II) complexes

Electronic structure of transition‐metal complexes. I. Parametrization for CNDO/2 method

Hydrogen bonding of flavoprotein: II. Effect of hydrogen bonding at hetero atoms of reduced flavin on its reactivity

Electronic structure of transition metal complexes. II. Trans influence in ammine complexes of the first transition metals.

Electronic structure of transition metal complexes. III. A comparative study of bondings in metal porphyrins.