Energy decomposition analysis of metal-metal bonding in [M2X8]2- (X=Cl, br) complexes of 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements

The electronic structures of a series of [M2X8]2- (X=Cl, Br) complexes involving 5f (U, Np, Pu), 5d (W, Re, Os), and 4d (Mo, Tc, Ru) elements have been calculated using density functional theory, and an energy decomposition approach has been used to carry out a detailed analysis of the metal-metal interactions. The energy decomposition analysis involves contributions from orbital interactions (mixing of occupied and unoccupied orbitals), electrostatic effects (Coulombic attraction and repulsion), and Pauli repulsion (associated with four-electron two-orbital interactions). As previously observed for Mo, W, and U M2X6 species, the general results suggest that the overall metal-metal interaction is considerably weaker or unfavorable in the actinide systems relative to the d-block analogues, as a consequence of a significantly more destabilizing contribution from the combined Pauli and electrostatic (prerelaxation) effects. Although the orbital-mixing (postrelaxation) contribution to the total bonding energy is predicted to be larger in the actinide complexes, this is not sufficiently strong to compensate for the comparatively greater destabilization originating from the Pauli-plus-electrostatic effects. A generally weak electrostatic contribution accounts for the large prerelaxation destabilization in the f-block systems, and ultimately for the weak or unfavorable nature of metal-metal bonding between the actinide elements. There is a greater variation in the energy decomposition results across the [M2Cl8]2- series for the actinide than for the d-block elements, both in the general behavior and in some particular properties.     

Determining relative f and d orbital contributions to M-Cl covalency in MCl6(2-) (M = Ti, Zr, Hf, U) and UOCl5(-) using Cl K-edge X-ray absorption spectroscopy and time-dependent density functional theory

Chlorine K-edge X-ray absorption spectroscopy (XAS) and ground-state and time-dependent hybrid density functional theory (DFT) were used to probe the electronic structures of O(h)-MCl(6)(2-) (M = Ti, Zr, Hf, U) and C(4v)-UOCl(5)(-), and to determine the relative contributions of valence 3d, 4d, 5d, 6d, and 5f orbitals in M-Cl bonding. Spectral interpretations were guided by time-dependent DFT calculated transition energies and oscillator strengths, which agree well with the experimental XAS spectra. The data provide new spectroscopic evidence for the involvement of both 5f and 6d orbitals in actinide-ligand bonding in UCl(6)(2-). For the MCl(6)(2-), where transitions into d orbitals of t(2g) symmetry are spectroscopically resolved for all four complexes, the experimentally determined Cl 3p character per M-Cl bond increases from 8.3(4)% (TiCl(6)(2-)) to 10.3(5)% (ZrCl(6)(2-)), 12(1)% (HfCl(6)(2-)), and 18(1)% (UCl(6)(2-)). Chlorine K-edge XAS spectra of UOCl(5)(-) provide additional insights into the transition assignments by lowering the symmetry to C(4v), where five pre-edge transitions into both 5f and 6d orbitals are observed. For UCl(6)(2-), the XAS data suggest that orbital mixing associated with the U 5f orbitals is considerably lower than that of the U 6d orbitals. For both UCl(6)(2-) and UOCl(5)(-), the ground-state DFT calculations predict a larger 5f contribution to bonding than is determined experimentally. These findings are discussed in the context of conventional theories of covalent bonding for d- and f-block metal complexes.     

Importance of energy level matching for bonding in Th(3+)-Am(3+) actinide metallocene amidinates, (C(5)Me(5))(2)[(i)PrNC(Me)N(i)Pr]An

The synthesis of a rare trivalent Th(3+) complex, (C(5)Me(5))(2)[(i)PrNC(Me)N(i)Pr]Th, initiated a density functional theory analysis on the electronic and molecular structures of trivalent actinide complexes of this type for An = Th, Pa, U, Np, Pu, and Am. While the 6d orbital is found to accommodate the unpaired spin in the Th(3+) species, the next member of the series, Pa, is characterized by an f(2) ground state, and later actinides successively fill the 5f shell. In this report, we principally examine the evolution of the bonding as one advances along the actinide row. We find that the early actinides (Pa-Np) are characterized by localized f orbitals and essentially ionic bonding, whereas the f orbitals in the later members of the series (Pu, Am) exhibit significant interaction and spin delocalization into the carbon- and nitrogen-based ligand orbitals. This is perhaps counter-intuitive since the f orbital radius and hence metal-ligand overlap decreases with increasing Z, but this trend is counter-acted by the fact that the actinide contraction also leads to a stabilization of the f orbital manifold that leads to a near degeneracy between the An 5f and cyclopentadienyl π-orbitals for Pu and Am, causing a significant orbital interaction.     

2,2'-Biphosphinines and 2,2'-bipyridines in homoleptic dianionic Group 4 complexes and neutral 2,2'-biphosphinine Group 6 d(6) metal complexes: octahedral versus trigonal-prismatic geometries

The geometric and electronic structure of formally d(6) tris-biphosphinine [M(bp)(3)](q) and tris-bipyridine [M(bpy)(3)](q) complexes were studied by means of DFT calculations with the B3LYP functional. In agreement with the available experimental data, Group 4 dianionic [M(bp)(3)](2-) complexes (1P-3P for M=Ti, Zr, and Hf, respectively) adopt a trigonal-prismatic (TP) structure, whereas the geometry of their nitrogen analogues [M(bpy)(3)](2-) (1N-3N) is nearly octahedral (OC), although a secondary minimum was found for the TP structures (1N'-3N'). The electronic factors at work in these systems are discussed by means of an MO analysis of the minima, MO correlation diagrams, and thermodynamic cycles connecting the octahedral and trigonal-prismatic limits. In all these complexes, pronounced electron transfer from the metal center to the lowest lying pi* ligand orbitals makes the d(6) electron count purely formal. However, it is shown that the bp and bpy ligands accommodate the release of electron density from the metal in different ways because of a change in the localization of the HOMO, which is a mainly metal-centered orbital in bp complexes and a pure pi* ligand orbital in bpy complexes. The energetic evolution of the HOMO allows a simple rationalization of the progressive change from the TP to the OC structure on successive oxidation of the [Zr(bp)(3)](2-) complex, a trend in agreement with the experimental structure of the monoanionic complex. The geometry of Group 6 neutral complexes [M(bp)(3)] (4P and 5P for M=Mo and W, respectively) is found to be intermediate between the TP and OC limits, as previously shown experimentally for the tungsten complex. The electron transfer from the metal center to the lowest lying pi* ligand orbitals is found to be significantly smaller than for the Group 4 dianionic analogues. The geometrical change between [Zr(bp)(3)](2-) and [W(bp)(3)] is analyzed by means of a thermodynamic cycle and it is shown that a larger ligand-ligand repulsion plays an important role in favoring the distortion of the tungsten complex away from the TP structure.     

Electronic structure of 3d[M(H2O)6](3+) ions from Sc(III) to Fe(III): a quantum mechanical study based on DFT computations and natural bond orbital analyses

The metal-donor atom bonding along the series of 3d[M(H2O)6](3+) ions from Sc(3+) to Fe(3+) has been investigated by density-functional calculations combined with natural localized bond orbital analyses. The M-OH(2) bonds were considered as donor-acceptor bonds, and the contributions coming from the metal ion's 3d sigma-, 3d pi-, and 4s sigma-interactions were treated individually. In this way, the total amount of charge transferred from the water oxygen-donor atoms toward the appropriate metal orbitals could be analyzed in a straightforward manner. One result obtained along these lines is that the overall extent of ligand-to-metal charge transfer shows a strong correlation to the hydration enthalpies of the aqua metal ions. If the contributions to the total ligand-to-metal ion charge transfer are divided into sigma- and pi-contributions, it turns out that Cr(3+) is the best sigma-acceptor, but its pi-accepting abilities are the weakest along the series. Fe(3+) is found to be the best pi-acceptor among the 3d hexaaqua ions studied. Its aptitude to accept sigma-electron density is the second weakest along the series and only slightly higher than that of Sc(3+) (the least sigma-acceptor of all ions) because of the larger involvement of the Fe(3+) 4s orbital in sigma-bonding. The strengths of the three types of bonding interactions have been correlated with the electron affinities of the different metal orbitals. Deviations from the regular trends of electron affinities along the series were found for those [M(H2O)6](3+) ions that are subject to Jahn-Teller distortions. In these cases (d(1) = [Ti(H2O)6](3+), d(2) = [V(H2O)6](3+), and d(4) = [Mn(H2O)6](3+)), ligand-to-metal charge transfer is prevented to go into those metal orbitals that contain unpaired d electrons. A lowering of the complex symmetry is observed and coupled with the following variations: The Ti(3+)- and V(3+)-hexaaqua ions switch from T(h)() to C(i)() symmetry while the Mn(3+)-hexaaqua ion moves to D(2)(h)() symmetry. The loss of orbital overlap leading to a diminished ligand-to-metal charge transfer toward the single occupied metal orbitals is compensated by amplified bonding interactions of the ligand orbitals with the unoccupied metal orbitals to some extent.     

Periodic trends and complexation chemistry of tetravalent actinide ions with a potential actinide decorporation agent 5-LIO(Me-3,2-HOPO): A relativistic density functional theory exploration

A relativistic density functional theory (DFT) study is reported which aims to understand the complexation chemistry of An⁴⁺ ions (An = Th, U, Np, and Pu) with a potential decorporation agent, 5-LIO(Me-3,2-HOPO). The calculations show that the periodic change of the metal binding free energy has an excellent correlation with the ionic radii and such change of ionic radii also leads to the structural modulation of actinide–ligand complexes. The calculated structural and binding parameters agree well with the available experimental data. Atomic charges derived from quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analysis shows the major role of ligand-to-metal charge transfer in the stability of the complexes. Energy decomposition analysis, QTAIM, and electron localization function (ELF) predict that the actinide–ligand bond is dominantly ionic, but the contribution of orbital interaction is considerable and increases from Th⁴⁺ to Pu⁴⁺. A decomposition of orbital contributions applying the extended transition state-natural orbital chemical valence method points out the significant π-donation from the oxygen donor centers to the electron-poor actinide ion. Molecular orbital analysis suggests an increasing trend of orbital mixing in the context of 5f orbital participation across the tetravalent An series (Th-Pu). However, the corresponding overlap integral is found to be smaller than in the case of 6d orbital participation. An analysis of the results from the aforementioned electronic structure methods indicates that such orbital participation possibly arises due to the energy matching of ligand and metal orbitals and carries the signature of near-degeneracy driven covalency.     

Metal-metal bonding in molecular actinide compounds: electronic structure of [M2X8](2-) (M = U, Np, Pu; X = Cl, Br, I) complexes and comparison with d-block analogues

Density functional and multiconfigurational (ab initio) calculations have been performed on [M(2)X(8)](2-) (X = Cl, Br, I) complexes of 4d (Mo, Tc, Ru), 5d (W, Re, Os), and 5f (U, Np, Pu) metals in order to investigate general trends, similarities and differences in the electronic structure and metal-metal bonding between f-block and d-block elements. Multiple metal-metal bonds consisting of a combination of sigma and pi interactions have been found in all species investigated, with delta-like interactions also occurring in the complexes of Tc, Re, Np, Ru, Os, and Pu. The molecular orbital analysis indicates that these metal-metal interactions possess predominantly d(z2) (sigma), d(xz) and d(yz) (pi), or d(xy) and d(x2-y2) (delta) character in the d-block species, and f(z3) (sigma), f(z2x) and f(z2y) (pi), or f(xyz) and f(z) (delta) character in the actinide systems. In the latter, all three (sigma, pi, delta) types of interaction exhibit bonding character, irrespective of whether the molecular symmetry is D(4h) or D(4d). By contrast, although the nature and properties of the sigma and pi bonds are largely similar for the D(4h) and D(4d) forms of the d-block complexes, the two most relevant metal-metal delta-like orbitals occur as a bonding and antibonding combination in D(4h) symmetry but as a nonbonding level in D(4d) symmetry. Multiconfigurational calculations have been performed on a subset of the actinide complexes, and show that a single electronic configuration plays a dominant role and corresponds to the lowest-energy configuration obtained using density functional theory.     

Computational study of analogues of the uranyl ion containing the -N=U=N- unit: density functional theory calculations on UO2(2+), UON+, UN2, UO(NPH3)3+, U(NPH3)2(4+), [UCl4[NPR3]2] (R = H, Me), and [UOCl4[NP(C6H5)3]

The electronic and geometric structures of the title species have been studied computationally using quasi-relativistic gradient-corrected density functional theory. The valence molecular orbital ordering of UO2(2+) is found to be pi g < pi u < sigma g < sigma u (highest occupied orbital), in agreement with previous experimental conclusions. The significant energy gap between the sigma g and sigma u orbitals is traced to the "pushing from below" mechanism: a filled-filled interaction between the semi-core uranium 6p atomic orbitals and the sigma u valence level. The U-N bonding in UON+ and UN2 is significantly more covalent than the U-O bonding in UON+ and UO2(2+). UO(NPH3)3+ and U(NPH3)2(4+) are similar to UO2(2+), UON+, and UN2 in having two valence molecular orbitals of metal-ligand sigma character and two of pi character, although they have additional orbitals not present in the triatomic systems, and the U-N sigma levels are more stable than the U-N pi orbitals. The inversion of U-N sigma/pi orbital ordering is traced to significant N-P (and P-H) sigma character in the U-N sigma levels. The pushing from below mechanism is found to destabilize the U-N f sigma molecular orbital with respect to the U-N d sigma level in U(NPH3)2(4+). The uranium f atomic orbitals play a greater role in metal-ligand bonding in UO2(2+), UN2, and U(NPH3)2(4+) than do the d atomic orbitals, although, while the relative roles of the uranium d and f atomic orbitals are similar in UO2(2+) and U(NPH3)2(4+), the metal d atomic orbitals have a more important role in the bonding in UN2. The preferred UNP angle in [UCl4(NPR3)2] (R = H, Me) and [UOCl4(NP(C6H5)3)]- is found to be close to 180 degrees in all cases. This preference for linearity decreases in the order R = Ph > R = Me > R = H and is traced to steric effects which in all cases overcome an electronic preference for bending at the nitrogen atom. Comparison of the present iminato (UNPR3) calculations with previous extended Hückel work on d block imido (MNR) systems reveals that in all cases there is little or no preference for linearity over bending at the nitrogen when R is (a) only sigma-bound to the nitrogen and (b) sterically unhindered. The U/N bond order in iminato complexes is best described as 3.     

Agostic interaction in the methylidene metal dihydride complexes H2MCH2 (M=Y, Zr, Nb, Mo, Ru, Th, or U)

Multiconfigurational quantum chemical methods (complete active space self-consistent field (CASSCF)/second-order perturbation theory (CASPT2)) have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U. The geometry of some of these complexes is highly irregular due to the formation of a three center bond CH...M, where the electrons in the CH bond are delocalized onto empty or half empty orbitals of d- or f-type on the metal. No agostic interaction is expected when M=Y, where only a single bond with methylene can be formed, or when M=Ru, because of the lack of empty electron accepting metal valence orbitals. The largest agostic interaction is found in the Zr and U complexes.     

Experimental and theoretical comparison of actinide and lanthanide bonding in M[N(EPR(2))(2)](3) complexes (M = U, Pu, La, Ce; E = S, Se, Te; R = Ph, iPr, H)

Treatment of M[N(SiMe3)2]3 (M = U, Pu (An); La, Ce (Ln)) with NH(EPPh2)2 and NH(EPiPr2)2 (E = S, Se), afforded the neutral complexes M[N(EPR2)2]3 (R = Ph, iPr). Tellurium donor complexes were synthesized by treatment of MI3(sol)4 (M = U, Pu; sol = py and M = La, Ce; sol = thf) with Na(tmeda)[N(TePiPr2)2]. The complexes have been structurally and spectroscopically characterized with concomitant computational modeling through density functional theory (DFT) calculations. The An-E bond lengths are shorter than the Ln-E bond lengths for metal ions of similar ionic radii, consistent with an increase in covalent interactions in the actinide bonding relative to the lanthanide bonding. In addition, the magnitude of the differences in the bonding is slightly greater with increasing softness of the chalcogen donor atom. The DFT calculations for the model systems correlate well with experimentally determined metrical parameters. They indicate that the enhanced covalency in the M-E bond as group 16 is descended arises mostly from increased metal d-orbital participation. Conversely, an increase in f-orbital participation is responsible for the enhancement of covalency in An-E bonds compared to Ln-E bonds. The fundamental and practical importance of such studies of the role of the valence d and f orbitals in the bonding of the f elements is emphasized.     

Out-of-center distortions in d(0) transition metal oxide fluoride anions

Electronic effects and the bond network are the two factors that cause out-of-center distortions in octahedral d(0) transition metal oxide fluoride anions. Overlap between filled oxide p orbitals and vacant cation d orbitals results in strong, short metal-oxide bonds causing the metal ion to distort toward the oxide ligand. This primary, electronic distortion is not dependent on the extended structure. Smaller, secondary distortions of the anionic octahedra are caused by interactions with the bond network. [HNC(6)H(6)OH](2)[Cu(NC(5)H(5))(4)(NbOF(5))(2)], prepared with 5-hydroxy-2-methylpyridine that provides two coordination contact sites to the anion when protonated, exhibits distortions in the anion reflecting both factors. Crystal data for [HNC(6)H(6)OH](2)[Cu(NC(5)H(5))(4)(NbOF(5))(2)]: monoclinic, space group C2/c (No. 15), with a = 10.9427(8) A, b = 16.204(1) A, c = 21.396(2) A, beta = 93.263(1) degrees, and Z = 4. Conditions for detection of both distortion types are discussed with five additional examples.     

Does covalency really increase across the 5f series? A comparison of molecular orbital, natural population, spin and electron density analyses of AnCp3 (An = Th-Cm; Cp = η(5)-C5H5)

The title compounds are studied with scalar relativistic, gradient-corrected (PBE) and hybrid (PBE0) density functional theory. The metal-Cp centroid distances shorten from ThCp(3) to NpCp(3), but lengthen again from PuCp(3) to CmCp(3). Examination of the valence molecular orbital structures reveals that the highest-lying Cp π(2,3)-based orbitals transform as 1e + 2e + 1a(1) + 1a(2). Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series such that in CmCp(3) the 5f manifold is at more negative energy than the Cp π(2,3)-based levels. Mulliken population analysis shows metal d orbital participation in the e symmetry Cp π(2,3)-based orbitals. Metal 5f character is found in the 1a(1) and 1a(2) levels, and this contribution increases significantly from ThCp(3) to AmCp(3). This is in agreement with the metal spin densities, which are enhanced above their formal value in NpCp(3), PuCp(3) and especially AmCp(3) with both PBE and PBE0. However, atoms-in-molecules analysis of the electron densities indicates that the An-Cp bonding is very ionic, increasingly so as the actinide becomes heavier. It is concluded that the large metal orbital contributions to the Cp π(2,3)-based levels, and enhanced metal spin densities toward the middle of the actinide series arise from a coincidental energy match of metal and ligand orbitals, and do not reflect genuinely increased covalency (in the sense of appreciable overlap between metal and ligand levels and a build up of electron density in the region between the actinide and carbon nuclei).     

Synthesis of a one-dimensional metal-dimer assembled system with interdimer interaction, M2(dtp)4 (M = Ni, Pd; dtp = dithiopropionato)

A metal-dimer assembled system, M(2)(dtp)(4) (M = Ni, Pd; dtp = dithiopropionate, C(2)H(5)CS(2-)), was synthesized and analyzed by the X-ray single-crystal diffraction method, UV-vis-near-IR spectra of solutions, solid-state diffuse reflectance spectroscopies, and electrical conductivity measurements. The structures exhibit one-dimensional metal-dimer chains of M(2)(dtp)(4) with moderate interdimer contact. These complexes are semiconducting or insulating, which is consistent with the fully filled d(z)2 band of M(II)(d(8)). Interdimer metal-metal distances were 3.644(2) Angstroms in Ni(2)(dtp)(4) and 3.428(2) Angstroms in Pd(2)(dtp)(4), each of which is marginally longer than twice the van der Waals radius of the metal. Interdimer charge-transfer transitions were nevertheless observed in diffuse reflectance spectra. The origin of this transition is considered to be due to an overlap of two adjacent d(sigma) orbitals, which spread out more than the d(z)2 orbital because of the antibonding d(sigma) character of the M(d(z)2)-M(d(z)2). The Ni(2)(dtp)(4) exhibited an interdimer charge-transfer band at a relatively low energy region, which is derived from the Coulomb repulsion of the 3d(sigma) orbital of Ni.     

Electronic structure of the [MNH2]+ (M = Sc-Cu) complexes

B3LYP geometry optimizations for the [MNH2]+ complexes of the first-row transition metal cations (Sc+-Cu+) were performed. Without any exception the ground states of these unsaturated amide complexes were calculated to possess planar geometries. CASPT2 binding energies that were corrected for zero-point energies and including relativistic effects show a qualitative trend across the series that closely resembles the experimental observations. The electronic structures for the complexes of the early and middle transition metal cations (Sc+-Co+) differ from the electronic structures derived for the complexes of the late transition metal cations (Ni+ and Cu+). For the former complexes the relative higher position of the 3d orbitals above the singly occupied 2p(pi) HOMO of the uncoordinated NH2 induces an electron transfer from the 3d shell to 2p(pi). The stabilization of the 3d orbitals from the left to the right along the first-row transition metal series causes these orbitals to become situated below the HOMO of the NH2 ligand for Ni+ and Cu+, preventing a transfer from occurring in the [MNH2]+ complexes of these metal cations. Analysis of the low-lying states of the amide complexes revealed a rather unique characteristic of their electronic structures that was found across the entire series. Rather exceptionally for the whole of chemistry, pi-type interactions were calculated to be stronger than the corresponding sigma-type interactions. The origin of this extraordinary behavior can be ascribed to the low-lying sp2 lone pair orbital of the NH2 ligand with respect to the 3d level.     

Activation of carbon-hydrogen bonds via 1,2-addition across M-X (X = OH or NH(2)) bonds of d(6) transition metals as a potential key step in hydrocarbon functionalization: a computational study

Recent reports of 1,2-addition of C-H bonds across Ru-X (X = amido, hydroxo) bonds of TpRu(PMe3)X fragments {Tp = hydridotris(pyrazolyl)borate} suggest opportunities for the development of new catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations, computational examinations of the efficacy of model d6 transition metal complexes of the form [(Tab)M(PH3)2X]q (Tab = tris-azo-borate; X = OH, NH2; q = -1 to +2; M = TcI, Re(I), Ru(II), Co(III), Ir(III), Ni(IV), Pt(IV)) for the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition states were located and found to have relatively little metal-hydrogen interaction. The C-H activation process is best described as a metal-mediated proton transfer in which the metal center and ligand X function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d6 electron configuration {especially the TabCo(PH3)2(OH)+ complex and related Co(III) systems} are predicted to be the most promising for further catalyst investigation.     

Diamagnetic group 6 tetrakis(di-tert-butylketimido)metal(IV) complexes

The addition of 4 equiv of LiN=C-t-Bu(2) to CrCl(3), MoCl(5), and WCl(6) in diethyl ether produced the complexes M(N=C-t-Bu(2))(4) (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual D(2d) symmetry in the solid state. (1)H and (13)C NMR spectra indicated that the complexes are diamagnetic, and a qualitative MO analysis showed that the orthogonal π-donor and -acceptor orbitals of the ketimide ligand cooperatively split the d(xy) and d(z2) orbitals sufficiently to allow spin pairing in the d(xy) orbital. A more sophisticated quantum-mechanical analysis of Cr(N=C-t-Bu(2))(4) using density functional/molecular mechanics methods confirmed the qualitative analysis by showing that the singlet state is 27 kcal/mol more stable than the triplet state.     

Metal vs ligand reduction in complexes of dipyrido[3,2-a:2',3'-c]phenazine and related ligands with [(C5Me5)ClM]+ (M = Rh or Ir): evidence for potential rather than orbital control in the reductive cleavage of the metal-chloride bond

Complexes between the chlorometal(III) cations [(C5Me5)ClM]+, M = Rh or Ir, and the 1,10-phenanthroline-derived alpha-diimine (N--N) ligands dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4,7,10-tetraazaphenanthrene (tap), or 1,10-phenanthroline-5,6-dione (pdo) were investigated by cyclic voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry with respect to either ligand-based or metal-centered (and then chloride-dissociative) reduction. Two low-lying unoccupied molecular orbitals (MOs) are present in each of these three N wedge N ligands; however, their different energies and interface properties are responsible for different results. Metal-centered chloride-releasing reduction was observed for complexes of the DNA-intercalation ligands dppz and tap to yield compounds [(N--N)(C5Me5)M] in a two-electron step. The separation of alpha-diimine centered optical orbitals and phenazine-based redox orbitals is apparent from the EPR and UV-vis-NIR spectroelectrochemistry of [(dppz)(C5Me5)M](0/*-/2-). In contrast, the pdo complexes undergo a reversible one-electron reduction to yield o-semiquinone radical complexes [(pdo)(C5Me5)ClM]* before releasing the chloride after the second electron uptake. The fact that the dppz complexes undergo a Cl(-)-dissociative two-electron reduction despite the presence of a lowest lying pi* MO (b1(phz)) with very little overlap to the metal suggests that an unoccupied metal/chloride-based orbital is lower in energy. This assertion is confirmed both by the half-wave reduction potentials of the ligands (tap, -1.95 V; dppz, -1.60 V; pdo, -0.85 V) and by the typical reduction peak potentials of the complexes [(L)(C5Me5)ClM](PF6) (tap, -1.1 V; dppz, -1.3 V; pdo, -0.6 V; all values against Fc(+/0)).     

Covalency in the f element-chalcogen bond. computational studies of M[N(EPR2)2]3 (M = La, Ce, Pr, Pm, Eu, U, Np, Pu, Am, Cm; E = O, S, Se, Te; R = H, (i)Pr, Ph)

The geometric and electronic structures of the title complexes have been studied using scalar relativistic, gradient-corrected density functional theory. Extension of our previous work on six-coordinate M[N(EPH 2) 2] 3 (M = La, Ce, U, Pu; E = O, S, Se, Te), models for the experimentally characterized M[N(EP (i)Pr 2) 2] 3, yields converged geometries for all of the other 4f and 5f metals studied and for all four group 16 elements. By contrast, converged geometries for nine-coordinate M[N(EPPh 2) 2] 3 are obtained only for E = S and Se. Comparison of the electronic structures of six- and nine-coordinate M[N(EPH 2) 2] 3 suggests that coordination of the N atoms produces only minor changes in the metal-chalcogen interactions. Six-coordinate Eu[N(EPH 2) 2] 3 and Am[N(EPH 2) 2] 3 with the heavier group 16 donors display geometric and electronic properties rather different from those of the other members of the 4f and 5f series, in particular, longer than expected Eu-E and Am-E bond lengths, smaller reductions in charge difference between M and E down group 16, and larger f populations. The latter are interpreted not as evidence of f-based metal-ligand covalency but rather as being indicative of ionic metal centers closer to M (II) than M (III). The Cm complexes are found to be very ionic, with very metal-localized f orbitals and Cm (III) centers. The implications of the results for the separation of the minor actinides from nuclear wastes are discussed, as is the validity of using La (III)/U (III) comparisons as models for minor actinide/Eu systems.     

Electronic structures and chemical bonding in transition metal monosilicides MSi (M = 3d, 4d, 5d elements)

Bond distances, vibrational frequencies, electron affinities, ionization potentials, dissociation energies, and dipole moments of the title molecules in neutral, positively, and negatively charged ions were studied using the density functional method. Ground state was assigned for each species. The bonding patterns were analyzed and compared with both the available data and across the series. It was found that besides an ionic component, covalent bonds are formed between the metal s,d orbitals and the silicon 3p orbital. The covalent character increases from ScSi (YSi) to NiSi (PdSi) for 3d (4d) metal monosilicides, then decreases. For 5d metal monosilicides, the covalent character increases from LaSi to OsSi, then decreases. For the dissociation of cations, the dissociation channel depends on the magnitude of the ionization potential between metal and silicon. If the ionization potential of the metal is smaller than that of silicon, channel MSi+ --> M+ + Si is favored. Otherwise, MSi+ --> M + Si+ will be favored. A similar behavior was observed for anions, in which the dissociation channel depends on the magnitude of electron affinity.     

"Non-VSEPR" Structures and Bonding in d(0) Systems

Under certain circumstances, metal complexes with a formal d(0) electronic configuration may exhibit structures that violate the traditional structure models, such as the VSEPR concept or simple ionic pictures. Some examples of such behavior, such as the bent gas-phase structures of some alkaline earth dihalides, or the trigonal prismatic coordination of some early transition metal chalcogenides or pnictides, have been known for a long time. However, the number of molecular examples for "non-VSEPR" structures has increased dramatically during the past decade, in particular in the realm of organometallic chemistry. At the same time, various theoretical models have been discussed, sometimes controversially, to explain the observed, unusual structures. Many d(0) systems are important in homogeneous and heterogeneous catalysis, biocatalysis (e.g. molybdenum or tungsten enzymes), or materials science (e.g. ferroelectric perovskites or zirconia). Moreover, their electronic structure without formally nonbonding d orbitals makes them unique starting points for a general understanding of structure, bonding, and reactivity of transition metal compounds. Here we attempt to provide a comprehensive view, both of the types of deviations of d(0) and related complexes from regular coordination arrangements, and of the theoretical framework that allows their rationalization. Many computational and experimental examples are provided, with an emphasis on homoleptic mononuclear complexes. Then the factors that control the structures are discussed in detail. They are a) metal d orbital participation in sigma bonding, b) polarization of the outermost core shells, c) ligand repulsion, and d) pi bonding. Suggestions are made as to which of the factors are the dominant ones in certain situations. In heteroleptic complexes, the competition of sigma and pi bonding of the various ligands controls the structures in a complicated fashion. Some guidelines are provided that should help to better understand the interrelations. Bent's rule is of only very limited use in these types of systems, because of the paramount influence of pi bonding. Finally, computed and measured structures of multinuclear complexes are discussed, including possible consequences for the properties of bulk solids.