Donor Bridge Research Articles

Analogies between binuclear phospholyl and cyclopentadienyl manganese carbonyl complexes: seven-electron donor bridging phospholyl rings

Phosphacymantrene, (η5-C4H4P)Mn(CO)3, is a known stable compound and a potential precursor to binuclear (C4H4P)2Mn2(CO)n derivatives analogous to isovalent (C5H5)2Mn2(CO)n derivatives. In this connection the structures and energetics of binuclear derivatives have been investigated by density functional theory. The lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures have seven-electron donor η5,η1-C4H4P ligands bridging the two metal atoms, which are too far apart for a direct Mn–Mn bond. However, the lowest energy (C4H4P)2Mn2(CO)3 structure is predicted to be a triply carbonyl bridged structure with a formal MnMn triple bond of length 2.17 A analogous to (η5-C5H5)2Mn2(μ-CO)3, which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy (C4H4P)2Mn2(CO)2 structures have one terminal η5-C4H4P ring and one bridging seven-electron donor η5,η1-C4H4P ring. However, only slightly higher energy (C4H4P)2Mn2(CO)2 structures are found with two η5-C4H4P rings and thus analogous to previously found (η5-C5H5)2Mn2(CO)2 structures. Such a triplet (η5-C4H4P)2Mn2(CO)2 structure is predicted to have a formal MnMn triple bond of length 2.20 A. The two analogous singlet (η5-C4H4P)2Mn2(CO)2 structures have shorter MnMn bonds of length 2.07 A, which are interpreted as the formal quadruple bonds required to give each Mn atom the favored 18-electron configuration.

Geometrical effect of stilbene on the performance of organic dye-sensitized solar cells

New metal-free organic donor–bridge–acceptor dyes comprising a triphenylamine moiety as the electron donor (D), a cyanoacrylic acid moiety as the electron acceptor (A), and a conjugated chromophore between D and A as a spacer (S) were synthesized for the application of dye-sensitized solar cells (DSSCs). Two types of S were designed, one contains a normal stilbene backbone (dye 1) and the other contains a 2-phenylindene moiety (dye 2) with the central double bond locked in a transoid geometry. The performance of DSSCs made with these two types of dyes was compared, and it was found that the inhibition of trans/cis isomerization of the central double bond was advantageous to the efficiency of DSSCs. The phenomenon was investigated by a comparison of their photophysical properties, and rationalized by quantum mechanical calculations by using density functional theory (DFT) at the B3LYP/6-31G(d,p) level. The electronic transitions were analyzed by time-dependent DFT, and the results were consistent with the experimental observations. The DSSCs were fabricated using both types of materials, and the performance was recorded under AM 1.5G irradiation (100 mW cm−2) condition. The overall conversion efficiencies of dye 2 (5.14–5.67%) were higher than those of dye 1 (4.52–4.98%). Both the short-circuit photocurrent density (Jsc) and the open-circuit voltage (Voc) were improved in 2 with respect to those of 1.

Impact of concentration self-quenching on the charge generation yield of fullerene based donor–bridge–acceptor compounds in the solid state

A fullerene based Donor-Bridge-Acceptor (DBA) compound, incorporating a π-extended tetrathiafulvalene electron donor, is investigated with respect to its photophysics in solution versus solid state. Solid films of neat DBA are compared with blend films where the DBA compound is diluted in the inert, low dielectric, polymer poly(styrene). It is found that the moderate intermolecular electronic coupling and donor-acceptor separation (22 Å) in this case leads to the generation of more dissociated, intermolecular charges than a mixture of the donor and acceptor reference compounds. However, the increased intermolecular interactions in the solid state lead to the excited state of the fullerene suffering from concentration self-quenching. This is found to severely affect the charge generation yield in solid films. The impact of competing intra and intermolecular interactions in the solid state upon the film photophysics is analysed in terms of a kinetic model which includes both the effects of concentration self-quenching and the impact of film composition upon the dielectric stabilisation of charge separated states. We conclude that both concentration self-quenching and dielectric stabilisation are critical in determining the photophysics of the blend films, and discuss strategies based upon our observations to enhance the charge photogeneration properties of organic films and photovoltaic devices based upon DBA compounds.

New stable donor–acceptor dyads for molecular electronics

A series of new donor–bridge–acceptor dyads with high chemical, electrochemical, thermal and conformational stability were synthesized by Stille coupling of oligo(3,4-ethylenedioxy)thiophenes (nEDOTs) and 1,4,5,8-naphthalenetetracarboxydiimide (NDI) building blocks. The molecular design provides for complete separation of HOMO and LUMO orbitals. A thiol functionality allows for selective anchoring of the dyads to metal electrodes, through either the donor or acceptor sides of the molecule. The optoelectronic properties of the dyads, both in solution and in self-assembled monolayers on gold, were characterized by electrochemistry, spectroelectrochemistry and UV-Vis absorption/emission spectroscopy and the results were further supported by DFT calculations.

Dependence of nonadiabatic intramolecular dissociative electron transfers on stereochemistry and driving force

The intramolecular dissociative electron transfer (ET) across donor–bridge–acceptor (D–B–A) systems consisting of a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,N-dimethylformamide has been studied by cyclic voltammetry. X-ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated D–B–A molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation. As previously found with the corresponding cis equatorial, axial conformers (Antonello and Maran, 1998 [25]), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the C–Br bond (acceptor A) through the 1,4-cyclohexanediyl spacer (bridge B). The intramolecular ETs are exergonic with driving force in the range from −0.5 to −1.2eV. The electrode process follows the same mechanism previously established for the cis series of isomers, but the ET rate constants obtained with the trans axial, axial isomers are larger by 1.1 orders of magnitude. X-ray crystallography structures show that the rate increase cannot be ascribed to a simple decrease of the geometrical distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the German–Kuznetsov theory of nonadiabatic dissociative ET (German and Kuznetsov, 1994 [35]) shows that the rate increase is caused by a more favorable coupling between the electronic wave functions describing the reagent and product states. The data and trends are discussed in comparison with other nonadiabatic intramolecular dissociative ET processes.

Self-assembled tetranuclear metallacyclic chair using orthogonal tritopic acceptors, angular ditopic donors, and bischelating bridging motifs

Abstract Self-assembled neutral tetrarhenium molecular chair was prepared using orthogonal tritopic acceptors, angular ditopic N-donors (~ 80°), and linear bis-chelating bridging donors in a one-step process.

Boronyl Ligand as a Member of the Isoelectronic Series BO− → CO → NO+: Viable Cobalt Carbonyl Boronyl Derivatives?

Recently the first boronyl (oxoboryl) complex [(c-C(6)H(11))(3)P](2)Pt(BO)Br was synthesized. The boronyl ligand in this complex is a member of the isoelectronic series BO(-) → CO → NO(+). The cobalt carbonyl boronyls Co(BO)(CO)(4) and Co(2)(BO)(2)(CO)(7), with cobalt in the formal d(8) +1 oxidation state, are thus isoelectronic with the familiar homoleptic iron carbonyls Fe(CO)(5) and Fe(2)(CO)(9). Density functional theory predicts Co(BO)(CO)(4) to have a trigonal bipyramidal structure with the BO group in an axial position. The tricarbonyl Co(BO)(CO)(3) is predicted to have a distorted square planar structure, similar to those of other 16-electron complexes of d(8) transition metals. Higher energy Co(BO)(CO)(n) (n = 3, 2) structures may be derived by removal of one (for n = 3) or two (for n = 2) CO groups from a trigonal bipyramidal Co(BO)(CO)(4) structure. Structures with a CO group bridging 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units and no Co-Co bond are found for Co(2)(BO)(2)(CO)(8). However, Co(2)(BO)(2)(CO)(8) is not viable because of the predicted exothermic loss of CO to give Co(2)(BO)(2)(CO)(7). The lowest lying Co(2)(BO)(2)(CO)(7) structure is a triply bridged (2BO + CO) structure closely related to the experimental Fe(2)(CO)(9) structure. However, other relatively low energy Co(2)(BO)(2)(CO)(7) structures are found, either with a single CO bridge, similar to the experimental Os(2)(CO)(8)(μ-CO) structure; or with 17-electron Co(CO)(4) and Co(BO)(2)(CO)(3) units joined by a single Co-Co bond with or without semibridging carbonyl groups. Both triplet and singlet Co(2)(BO)(2)(CO)(6) structures are found. The lowest lying triplet Co(2)(BO)(2)(CO)(6) structures have a Co(CO)(3)(BO)(2) unit coordinated to a Co(CO)(3) unit through the oxygen atoms of the boronyl groups with a non-bonding ∼4.3 Å Co···Co distance. The lowest lying singlet Co(2)(BO)(2)(CO)(6) structures have either two three-electron donor bridging η(2)-μ-BO groups and no Co···Co bond or one such three-electron donor BO group and a formal Co-Co single bond.

Photoinduced Processes in Fluorene‐Bridged Rhenium–Phenothiazine Dyads – Comparison of Electron Transfer Across Fluorene, Phenylene, and Xylene Bridges

AbstractThe photoinduced processes occurring after pulsed laser excitation of a series of donor–bridge–acceptor molecules comprising a phenothiazine electron donor, variable‐length fluorene bridges, and a rhenium(I) electron acceptor were investigated. A dyad with a single fluorene bridge unit exhibits electron transfer from phenothiazine to the rhenium(I) complex upon photoexcitation, whereas in dyads with fluorene oligomers bridge‐localized triplet excited states are formed rather than electron transfer products. In the monofluorene‐bridged system with a donor–acceptor distance of ca. 15 Å, electron transfer occurs with a time constant of 1.9 ns. The equidistant electron transfer between the same donor and acceptor is considerably slower across a biphenyl bridge (3.9 ns) or a bi‐p‐xylene spacer (20 ns). This finding is interpreted in terms of different tunneling barrier heights associated with the charge transfer across the three different types of molecular bridges.

Binuclear Cyclopentadienylmanganese Carbonyl Thiocarbonyls: Four‐Electron Donor Bridging Thiocarbonyl Groups of Two Types and a Bridging Acetylenedithiolate Ligand

AbstractAs a starting point, the mononuclear cyclopentadienylmanganese carbonyl thiocarbonyls [CpMn(CS)(CO)n] (Cp = η5‐C5H5; n = 2, 1, 0) and binuclear derivatives [Cp2Mn2(CS)2(CO)n] (n = 3, 2, 1, 0) have been studied by density functional theory (DFT). For coordinately saturated binuclear [Cp2Mn2(CS)2(CO)3], the four electron donor end‐on CE(E = O, S) bridged structures are preferred energetically over the normal CE (E = O, S) bridged structures, analogous to [Cr2(CS)2(CO)9]. The lowest energy structure for [Cp2Mn2(CS)2(CO)2] has one four‐electron donor bridging η2‐μ‐CS group. However, this structure is thermodynamically unstable with respect to disproportionation into [Cp2Mn2(CS)2(CO)3] and [Cp2Mn2(CS)2(CO)]. Only two structures are found for [Cp2Mn2(CS)2(CO)] as is the case for the carbonyl analogue [Cp2Mn2(CO)3]. The global minimum of [Cp2Mn2(CS)2(CO)], a singlet triply bridged structure, is very favorable energetically with respect to loss of a CO group, disproportionation into [Cp2Mn2(CS)2(CO)2] + [(Cp)2Mn2(CS)2], and dissociation into mononuclear fragments. The lowest energy structure for [Cp2Mn2(CS)2] is a triplet structure with two bridging CS groups and the Mn≡Mn triple bond required to give each Mn atom a 17‐electron configuration for a binuclear triplet. A higher energy singlet [Cp2Mn2(CS)2] structure is found with a very short MnblablaMn distance of about 2.1 Å suggesting the formal quadruple bond required to give each Mn atom the favored 18‐electron configuration. In other higher energy singlet and triplet [Cp2Mn2(CS)2] structures the two CS ligands have coupled to form an acetylenedithiolate ligand bridging the two manganese atoms.

Binuclear fluoroborylene manganese carbonyls

The lowest energy structure for Mn 2(BF)(CO) 10 is predicted to consist of two Mn(CO) 5 groups bridged by a BF group without a manganese–manganese bond. This structure is related to the stable compounds Mn 2(μ-BX)(CO) 10 (X = Cl, Br) and [(η 5-C 5H 5)Ru(CO) 2] 2(μ-BF), which have been synthesized and characterized by X-ray crystallography. The following principles determine the lowest energy structures of the Mn 2(BF)(CO) n ( n = 9, 8, 7, 6) derivatives: (1) two-electron donor bridging μ-BF groups are highly favorable; (2) four-electron donor bridging η 2-μ-BF groups are not found and thus appear to be highly unfavorable. Thus the lowest energy structure of Mn 2(BF)(CO) 9 is a doubly bridged structure with bridging CO and BF groups, in contrast to the experimentally observed unbridged structure of Mn 2(CO) 10. The lowest energy structures of Mn 2(BF)(CO) 8 have either a four-electron donor η 2-μ-CO group or a two-electron donor bridging BF group. Similarly the lowest energy structures of the more highly unsaturated Mn 2(BF)(CO) n ( n = 7, 6) are singlet (for n = 7) or triplet (for n = 6) states in which the BF group is a bridging rather than a terminal ligand.

Ethical Considerations for Participation of Nondirected Living Donors in Kidney Exchange Programs

Kidneys from nondirected donors (NDDs) have historically been allocated directly to the deceased donor wait list (DDWL). Recently, however, NDDs have participated in kidney exchange (KE) procedures, including KE 'chains', which have received considerable media attention. This increasing application of KE chains with NDD participation has occurred with limited ethical analysis and without ethical guidelines. This article aims to provide a rigorous ethical evaluation of NDDs and chain KEs. NDDs and bridge donors (BDs) (i.e. living donors who link KE procedures within KE chains) raise several ethical concerns including coercion, privacy, confidentiality, exploitation and commercialization. In addition, although NDD participation in KE procedures may increase transplant numbers, it may also reduce NDD kidney allocation to the DDWL, and disadvantage vulnerable populations, particularly O blood group candidates. Open KE chains (also termed 'never-ending' chains) result in a permanent diversion of NDD kidneys from the DDWL. The concept of limited KE chains is discussed as an ethically preferable means for protecting NDDs and BDs from coercion and minimizing 'backing out', whereas 'honor systems' are rejected because they are coercive and override autonomy. Recent occurrences of BDs backing out argue for adoption of ethically based protective measures for NDD participation in KE.

Binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes with Schiff-bases derived from crown ether platforms: Rare examples of ether oxygen atoms bridging metal centers

Bibracchial lariat ethers L 3 and L 4 , derived from the condensation of N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 or N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 with salicylaldehyde, form binuclear complexes with Co(II), Ni(II), Cu(II) and Zn(II). Our studies show that the different denticity and crown moiety size of the two related receptors give rise to important differences on the structures of the corresponding complexes. Single crystal X-ray diffraction analysis shows that the [Ni 2(L 3)(H 2O) 2] 2+ and [Cu 2(L 3)(NO 3)] + complexes constitute a rare example in which an oxygen atom of the crown moiety is bridging the two six coordinate metal ions. In contrast, none of the oxygen atoms of the crown moiety is acting as a bridging donor atom in the [Co 2(L 4)(CH 3CN) 2] 2+, [Cu 2(L 4)] 2+ and [Zn 2(L 4)] 2+ complexes. This is attributed to the larger size the crown moiety and the higher denticity of L 4 compared to L 3 . In [Co 2(L 4)(CH 3CN) 2] 2+ the metal ions show a distorted octahedral coordination, while in the Cu(II) and Zn(II) analogues the metal ions are five-coordinated in a distorted trigonal bipyramidal environment. In [Cu 2(L 3)(NO 3)] + the coordinated nitrate anion acts as a bidentate bridging ligand, which results in the formation of a 1D coordination polymer.

Comparing Spin‐Selective Charge Transport through Donor–Bridge–Acceptor Molecules with Different Oligomeric Aromatic Bridges

Burning bridges: The exponential distance dependence (β value) of singlet and triplet charge recombination (CR) pathways is determined for three donor-bridge-acceptor (DBA) molecules. p-Phenylethynylene and fluorenone bridges have similar β values, which differ significantly from those of p-phenylene bridges, thus implying that β for both singlet and triplet CR is system-dependent, not bridge-specific.

Synthesis and characterization of donor–bridge–acceptor alternating copolymers containing perylene diimide units and their application to photovoltaic cells

AbstractWe have used Suzuki coupling to prepare a series of alternating copolymers featuring coplanar cyclopentadithiophene and hole‐transporting carbazole units. We observed quenching in the photoluminescence spectra of our polymers after incorporating pendent electron‐deficient perylene diimide (PDI) moieties on the side chains, indicating more efficient photoinduced electron transfer. Electrochemical measurements revealed that the PDI‐containing copolymers displayed reasonable and sufficient offsets of the energy levels of their lowest unoccupied molecular orbitals for efficient charge dissociation. The performance of bulk heterojunction photovoltaic cells incorporating the copolymer/[6,6]‐phenyl‐C61‐butyric acid methyl ester blends (1:4, w/w) was optimized when the active layer had a thickness of 70 nm. The photocurrents of the devices were enhanced as a result of the presence of the PDI moieties, thereby leading to improved power conversion efficiencies. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1298–1309, 2010

Binuclear Nickel Carbonyl Thiocarbonyls: Metal−Metal Multiple Bonds versus Four-Electron Donor Thiocarbonyl Groups

The structures of the mononuclear derivatives Ni(CS)(CO)(n) (n = 3, 2, 1, 0) and the binuclear derivatives Ni(2)(CS)(2)(CO)(n) (n = 5, 4, 3, 2) have been optimized by density functional theory for comparison with the corresponding structures of Ni(CO)(n+1) and Ni(2)(CO)(n+2), respectively. In the lowest energy structures for Ni(CS)(CO)(n) (n = 3, 2, 1), the nickel atom has approximate tetrahedral (n = 3), trigonal planar (n = 2), or bent coordination (n = 1) corresponding to 18-, 16-, and 14-electron metal configurations, respectively. The six lowest energy Ni(2)(CS)(2)(CO)(5) structures all have a single CE (E = S, O) bridge and a formal Ni-Ni single bond of length approximately 2.6 to approximately 2.7 A analogous to the lowest energy Ni(2)(CO)(7) structure. The Ni(2)(CS)(2)(CO)(5) structures with a bridging CS group are of lower energies than similar structures with a bridging CO group. Higher energy Ni(2)(CS)(2)(CO)(5) structures with a linear bridging CE group (E = O, S) and no Ni...Ni bond are also found with tetrahedral coordination for both nickel atoms. The lowest energy Ni(2)(CS)(2)(CO)(4) structures are doubly bridged structures with only two-electron donor CO and CS groups and Ni=Ni distances of approximately 2.5 A suggesting the formal double bond needed to give both nickel atoms the favored 18-electron configuration. For Ni(2)(CS)(2)(CO)(3) the structures with one four-electron donor bridging eta(2)-mu-CS group and a formal Ni=Ni double bond of approximately 2.4 to approximately 2.5 A are energetically preferred over triply bridged structures with a shorter Ni[triple bond]Ni distance of approximately 2.2 A, corresponding to a formal triple bond and similar to the lowest energy Ni(2)(CO)(5) structure. The lowest energy Ni(2)(CS)(2)(CO)(2) structures are generally derived from Ni(2)(CS)(2)(CO)(3) structures by removal of a carbonyl group. No formal quadruple bonds are found in any of the Ni(2)(CS)(2)(CO)(2) structures, as indicated by the absence of ultrashort Ni(-4)Ni distances.

Enhancement of charge transition from donor to acceptor in organic and biochemical reactions by the intermediate-mode-assisted tunneling mechanism

The intermediate-mode-assisted tunneling mechanism is shown to be relevant for the control of electron (/hole/energy) transfer in donor–bridge–acceptor based symmetric molecular electronic systems. In this case the donor and the acceptor are superpositions of two practically degenerate eigenstates of the system. A molecular eigenstate of the system, which is mainly localized on the bridge, serves as intermediate state. Upon a small symmetry preserving variation of the bridge site geometry this intermediate state is coupled only to one of the two practically degenerate states. Our approach gives the tools to design a molecular switch where 100% ultrafast electron/hole transition occurs. As an illustrative numerical example the electron/hole transition in a cation radical of a polycyclic analogue of norbornadienone (PC-NBDO) molecule is calculated. We observe that, because of a sudden stretch in the CO bond of the bridge site, the electron transfer time has been decreased from infinity to 11 femtoseconds.

Unsaturation and Variable Hapticity in Binuclear Azulene Iron Carbonyl Complexes

The bicyclic hydrocarbon azulene has long been known to form a variety of metal carbonyl derivatives with diverse metals. In this connection the binuclear azulene iron carbonyls C10H8Fe2(CO)n (n = 6, 5, 4, 3, 2, 1) have been investigated herein. For C10H8Fe2(CO)6 the singlet cis and trans structures with the two iron atoms on the same side or opposite sites of the azulene ring system were found to be of comparable energies. The lowest energy structure for C10H8Fe2(CO)5 is the experimentally known singlet cis-η5,η3-C10H8Fe2(CO)5 structure with the C5 ring bonded to the Fe(CO)2 group as a pentahapto ligand and the C7 ring bonded to the Fe(CO)3 group as a trihapto ligand and with a formal Fe−Fe single bond length of 2.83 ± 0.05 Å. This structure is thermodynamically stable with respect to disproportionation into C10H8Fe2(CO)6 + C10H8Fe2(CO)4 by 31 ± 3 kcal/mol and resistant toward CO dissociation by 45 ± 3 kcal/mol, consistent with the fact that it is readily obtained in the laboratory by simple thermal reactions of azulene with iron carbonyls. The lowest energy C10H8Fe2(CO)4 structure is also a singlet cis-η5,η3-C10H8Fe2(CO)4 structure. This compound is predicted to have either a four-electron donor bridging η2-μ-CO group and a formal Fe−Fe single bond of length 2.73 Å by B3LYP or, more likely, all two-electron donor carbonyl groups and a formal Fe═Fe double bond of length 2.48 Å by BP86. The lowest energy structures of the more highly unsaturated C10H8Fe2(CO)n (n = 3, 2, 1) are triplet (n = 3) or quintet (n = 2, 1) spin states with both iron atoms on the same side of the azulene system. In these systems all 10 carbon atoms of the azulene system are within bonding distance of an iron atom with a short enough iron−iron distance to suggest multiple bonding. The lowest energy C10H8Fe2(CO)n (n = 4, 3) structures are predicted to be either thermodynamically unstable or marginally stable with respect to disproportionation into C10H8Fe2(CO)n+1 + C10H8Fe2(CO)n−1. This again is consistent with the experimental observation of cis-C10H8Fe2(CO)5 as the major product from the thermal reaction of azulene with iron carbonyls.

Possibilities for Titanium−Titanium Multiple Bonding in Binuclear Cyclopentadienyltitanium Carbonyls: 16-Electron Metal Configurations and Four-Electron Donor Bridging Carbonyl Groups as Alternatives

The structures for the binuclear Cp(2)Ti(2)(CO)(n) derivatives (Cp = eta(5)-C(5)H(5); n = 8, 7, 6, 5, 4, 3, 2) have been optimized using density functional theory. Furthermore, the thermodynamics of CO dissociation, disproportionation into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1), and dissociation into mononuclear fragments of these Cp(2)Ti(2)(CO)(n) derivatives have been studied. An unbridged Cp(2)Ti(2)(CO)(8) structure with a long approximately 3.9 A Ti-Ti bond is found. As expected from the long Ti-Ti bond, the predicted dissociation energy of this dimer into CpTi(CO)(4) fragments is relatively low at 7 +/- 3 kcal/mol. The lowest energy Cp(2)Ti(2)(CO)(6) structure has two CpTi(CO)(3) units linked by a formal approximately 2.8 A Ti identical withTi triple bond and thus is the next member of the M identical withM triply bonded series Cp(2)V(2)(CO)(5), Cp(2)Cr(2)(CO)(4), Cp(2)Mn(2)(CO)(3), all three of which are stable compounds. The lowest energy structures of Cp(2)Ti(2)(CO)(7), Cp(2)Ti(2)(CO)(5), and Cp(2)Ti(2)(CO)(4) all contain one or two four-electron donor bridging eta(2)-mu-CO groups. However, they are not likely to be stable molecules since their disproportionation energies into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1) are either nearly thermoneutral (n = 5) or exothermic (n = 7 and 4). The lowest energy structure of Cp(2)Ti(2)(CO)(3), in which all three carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a approximately 3.05 A formal Ti-Ti single bond, is a promising synthetic target since it is thermodynamically stable with respect to both CO dissociation and disproportionation into Cp(2)Ti(2)(CO)(4) + Cp(2)Ti(2)(CO)(2). In the lowest energy Cp(2)Ti(2)(CO)(2) structure both carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a formal 2.74 A Ti[triple bond]Ti triple bond. These low energy Cp(2)Ti(2)(CO)(n) (n = 3, 2) structures have only a 16-electron titanium configuration rather than the usually favorable 18-electron configuration for metal carbonyl complexes.

Excitation energy transfer in donor–bridge–acceptor systems

This perspective will focus on the mechanistic aspects of singlet and triplet excitation energy transfer. Well defined donor-bridge-acceptor systems specifically designed for investigating the distance and energy gap dependencies of the energy transfer reactions are discussed along with some recent developments in computational modeling of the electronic coupling.

Binuclear manganesecarbonyl thiocarbonyls: metal–metal multiple bonds versus four-electron donorthiocarbonyl groups

Density functional theory (DFT) studies on Mn2(CS)2(CO)8 using the B3LYP and BP86 methods show that no less than eight different unbridged structures are of significantly lower energies than the lowest energy doubly bridged structure. The Mn–Mn single bonds in these Mn2(CS)2(CO)8 structures range from 2.99 ± 0.02 A for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 A for the four structures with eclipsed equatorial CO/CS groups. The six lowest energy Mn2(CS)2(CO)7 structures all have four-electron donor bridging η2-μ-CE groups (E = S, O) and formal Mn–Mn single bonds of lengths 2.95 ± 0.01 A, rather than only two-electron donor CO and CS groups and formal MnMn double bonds. The Mn2(CS)2(CO)7 structures with an η2-μ-CS group are of lower energy than those with an η2-μ-CO group. These Mn2(CS)2(CO)7 structures are similar to the lowest energy structure for Mn2(CO)9 predicted previously as well as (Ph2PCH2PPh2)2Mn2(CO)4(η2-μ-CO), which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy Mn2(CS)2(CO)6 structures are predicted have a single four-electron donor bridging η2-μ-CS group and a formal MnMn double bond. However, at only slightly higher energies, Mn2(CS)2(CO)6 structures are found with two η2-μ-CS groups and a formal Mn–Mn single bond. A formal MnMn triple bond of length 2.36 ± 0.03 A is found in an even higher energy unbridged Mn2(CS)2(CO)6 structure, similar to the lowest energy Mn2(CO)8 structure found in a previous theoretical study. The lowest energy structures for Mn2(CS)2(CO)5 have two η2-μ-CS groups and a formal MnMn double bond of length 2.57 ± 0.03 A.