Simple Transition-metal Complexes Research Articles

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High‐energy relatives of benzene have captured the imagination of chemists, especially those with cumulative double bonds which feature strained ring structure and lack any aromaticity stabilization. The integration of transition metals and Group 14 elements into six‐membered cycloallene rings has been achieved by a [5+1]‐cycloaddition of diethynylsilane or diethynylgermane with simple transition metal complexes. Experimental and computational studies demonstrate the stability of these unprecedent cycloallenes, i.e. metalla‐isosilabenzenes and metalla‐isogermabenzenes, may root in the decreasing strain on the cumulative double bonds. More details are discussed in the article by Zhang et al. on page 1121—1127.image

Spin Splitting Energy of Transition Metals: A New, More Affordable Wave Function Benchmark Method and Its Use to Test Density Functional Theory.

Accurately predicting the spin splitting energy of chemical species is important for understanding their reactivity and magnetic properties, but it is very challenging, especially for molecules containing transition metals. One impediment to progress is the scarcity of accurate benchmark data. Here we report a set of calculations designed to yield reliable benchmarks for simple transition-metal complexes that can be used to test density functional methods that are affordable for large systems of more practical interest. Various wave function methods are tested against experiment for Fe2+, Fe3+, and Co3+, including CASSCF, CASPT2, CASPT3, MRCISD, MRCISD+Q, ACPF, AQCC, CCSD(T), and CASPT2/CCSD(T) and also a new method called CASPT2.5, which is performed by taking the average of the CASPT2 and CASPT3 energies. We find that MRCISD+Q, ACPF, and AQCC require smaller active spaces for good accuracy than are required by CASPT2 and CASPT3, and this aspect may be important for calculations on larger molecules; here we find that CASPT2.5 extrapolated to a complete basis set is the most suitable method-in terms of computational cost and in terms of accuracy on monatomic systems-and therefore we chose this method for molecular benchmarks. Then Kohn-Sham density functional calculations with 60 exchange-correlation functionals are tested for FeF2, FeCl2, and CoF2. We find that MN15-L, M06-SX, and revM06 have very good agreement with CASPT2.5 benchmarks in terms of both the spin splitting energy and the optimized geometry for each spin state. In addition, we recommend def2-TZVP as the most suitable basis set to perform density functional calculations for molecular spin splitting energies; extra polarization functions in the basis set do not help to increase the accuracy of the spin splitting energy in KS calculations.

Pattern-based sensing with simple metal–dye complexes

Different strategies for the creation of optical sensors with metal-dye complexes are discussed. The focus is on sensors, which are used in conjunction with pattern recognition protocols. It is shown that remarkably powerful sensors can be obtained by combining commercially available dyes with simple transition metal complexes.

Selective Functionalization of Alkanes by Transition-Metal Boryl Complexes

Simple transition-metal complexes of the formula Cp*M(CO) n BR 2 [Cp* = C 5 (CH 3 ) 5 ] containing an electrophilic, covalently bound main-group ligand react with alkanes and release products resulting from selective functionalization of an alkane at the terminal position. These reactions produce alkylboronate esters, which are common reagents in organic synthesis. Thus, the boryl complexes are rare chemical reagents that react selectivity at the terminal position of an alkane to provide simple functionalized products. Mechanistic analysis shows that ligand dissociation is induced photochemically and that thermal reaction of the resulting intermediate occurs with alkanes.

ChemInform Abstract: Dihydrogen Complexes of the Transition Metals

The binding and activation of dihydrogen by simple transition metal complexes is of fundamental importance in processes as diverse as the homogeneous or heterogeneous hydrogenation of unsaturated organic molecules(1) and understanding how metalloenzymes such as nitrogenase(2) and the hydrogenases(3) work at the atomic level. Simple consideration of the oxidative-addition of dihydrogen to a coordinatively-unsaturated complex {or its reverse (reductive-elimination)} reveals that the reactions are compelled to proceed via a dihydrogen complex as shown in Equation (1). However until recently it was considered that the dihydrogen complex had only a fleeting existence. Although there had been some reports in the literature such as that by Ashworth and Singleton(4) that the formally RuIV trihydride, [RuH3(PMe2Ph2]+ was better formulated as the Ru11 species [RuH(H2)(PMe2Ph)2]+, these could not be substantiated. In 1984, though, Kubas showed that the apparently innocuous complex [WH2(CO)3(PPr 3 i )2] contained the side-on bonded dihydrogen molecule, established unambiguously by x-ray, and neutron crystallography and spectroscopy(5). In this highlight the current status of dihydrogen complexes, their structure, identification and in particular their reactivity will be discussed.

Dihydrogen complexes of the transition metals

The binding and activation of dihydrogen by simple transition metal complexes is of fundamental importance in processes as diverse as the homogeneous or heterogeneous hydrogenation of unsaturated organic molecules(1) and understanding how metalloenzymes such as nitrogenase(2) and the hydrogenases(3) work at the atomic level. Simple consideration of the oxidative-addition of dihydrogen to a coordinatively-unsaturated complex {or its reverse (reductive-elimination)} reveals that the reactions are compelled to proceed via a dihydrogen complex as shown in Equation (1). However until recently it was considered that the dihydrogen complex had only a fleeting existence. Although there had been some reports in the literature such as that by Ashworth and Singleton(4) that the formally RuIV trihydride, [RuH3(PMe2Ph2]+ was better formulated as the Ru11 species [RuH(H2)(PMe2Ph)2]+, these could not be substantiated. In 1984, though, Kubas showed that the apparently innocuous complex [WH2(CO)3(PPr 3 i )2] contained the side-on bonded dihydrogen molecule, established unambiguously by x-ray, and neutron crystallography and spectroscopy(5). In this highlight the current status of dihydrogen complexes, their structure, identification and in particular their reactivity will be discussed.

Localized molecular orbitals of simple metal-olefin complexes

Localized molecular orbitals (LMOs) for a series of simple transition-metal complexes are presented. The relationship between the LMO bonding patterns found and the symmetry based description of the bonding is discussed.

Diffraction experiments and the theory of simple transition-metal complexes

Abstract Polarized neutron and X-ray diffraction experimental results on simple Cr(III), Ni(II) and Co(ll) complexes are compared with theoretical calculations. A simple ionic (crystal-field) model is a useful first approximation for the description of the spin and charge densities. For the more covalently bound cases the effects of electron-electron correlation in the metal-ligand bonding are as important as the spin and charge transferred via simple covalence. Thus simple M.O. models do not offer much improvement, nor do ab initio calculations which do not include configurational interaction (viz. restricted Hartree-Fock). Unconstrained calculations which partly incorporate electron-electron correlation are in qualitative, but not quantitative, agreement with the experiments. The polarized neutron diffraction experiments provide such sensitive tests of theory because they spatially separate the small effects in the bonding region from the metal atom 3d region, whereas these are inextricably mixed in the...

Theoretical study of cis-trans isomerization in simple transition-metal complexes

PRDDO calculations on several simple octahedral transition-metal complexes are presented. Relative energies, optimized ge- ometries, and population analyses are discussed for a series of complexes of general formulas M(NH3)4(C0)2 and M(NH3)2(C0)4, M = Cro, Mn+, Fe2+, and Co. Back-bonding stabilizes the cis isomers of the chromium and manganese complexes, but not those of Fe2+ or Cos+. Similar calculations on Cr(CO)2(N2)4 and Cr(CO),(N,), also predict the cis isomers to be favored energetically and are consistent with available experimental evidence. The cis isomers of Cr(C0)2(PH3)4 and Cr(CO),(PH,), are both favored by less than 0.3 kcal/mol, even though the cis conformations of the corresponding amine complexes are strongly favored (6-13 kcal/mol). As reported previously, PH3 can act as a T acceptor even without d orbitals on phosphorus, the acceptor orbital being best described as P-H u*. The energetic manifestation of this effect is a reduction of the cis-trans isomerization energy. Finally, calculations on the conformational preferences of bis(pyridine)bis(acetylacetonato)metal complexes are discussed. The phenomenon of r bonding in organometallic compounds has been studied intensely from both experimental and theoretical perspectives. Some of the most obvious and important conse- quences of r-bonding effects are in the areas of site preferences, conformational stability, and relative bond lengths. A good ex- ample of how r bonding can influence site preference may be found in Hoffmann's work on transition-metal pentacoordination, where the site preferences of r-bonding ligands are detailed for trigonal-bipyramidal and square-pyramidal structures.' Con- formational preferences due to r bonding have been studied in a variety of systems.2 Experimental studies of the effects of r bonding on bond lengths abound in the literature, but some of the clearest results are found in chromium-carbonyl complexes with pho~phine,~ where a distinct trend of long metal-ligand bond lengths trans to strong .rr acceptors can be seen. Here, we present a theoretical study of conformational preferences and relative bond lengths in an isoelectronic and isostructural series of complexes of chromium, manganese, iron, and cobalt. Consider a (hypothetical) complex such as CT(NH~)~(CO)~. The pseudooctahedral coordination leads to the usual d-orbital splittings, with three occupied tp orbitals at low energy and two empty eg orbitals at higher energy (here and throughout this paper we employ octahedral symmetry notation for simplicity). Simple r-bonding arguments lead one to predict that this complex should exist in the cis form, since only that conformation allows r back-bonding to the carbonyls from all three tzg orbitals. Similar conclusions can be reached for systems such as Cr(NH3)4(C0)2 although here all three to orbitals are r bonding in both isomers. Nevertheless, the cis isomer reduces the number of mutually trans carbonyls and thus should be preferred. Indeed, in the absence of steric effects, mixed complexes of carbonyl and other non- or lesser-r-accepting ligands invariably assume the cis conformation. Examples are Cr(C0)2(PH3)4,3 Cr(C0)3(PH3)3,4 Cr(CO),(P- H3)2,' and MO(CO)~(PM~~)~,~ all of which exist in the cis form. AG for the cis-trans isomerization of the last compound is 0.32 kcal/mol, while more bulky phosphines are preferentially trans for steric reasom6 The effects of A bonding on the bond lengths of these and similar complexes are also clear. Thus, in ~is-cr(Co)~(PH~) ~, the Cr-P distance trans to CO is 2.34 A, while the corresponding distance trans to PH, is significantly shorter (2.28 A).3 In this paper, we study the geometric and energetic conse- quences of a bonding in a group of octahedral complexes of general

Simple Coordination Complexes: Drugs and Probes for DNA Structure

Abstract Specific interactions of some simple transition metal complexes with DNA are described. Several binding modes and reactions, intercalation, covalent modification, and oxidative cleavage, are discussed and all have in common the specific recognition by metal complexes of subtle features of DNA helical structure. The versatility of the metal center, from its spectroscopic properties to stereochemistry, suggests that coordination complexes may be promising candidates to exploit in the design of drugs and probes for DNA.