Cr Bond Research Articles

First-principles analysis of the adsorption of aluminum and chromium atoms on the HfC (001) surface

Abstract We have investigated the surface energies and electronic structures of the HfC (0 0 1) (1 × 1) surface. Calculations were also carried out for the adsorption of Al and Cr atoms on the surface. The structural and electronic properties of bulk HfC were also determined. All of these calculations were based on first principles. We have dealt with the exchange and correlation potential for bulk HfC using the Perdew–Wang generalized gradient approximation (GGA-PW91) (1991). Furthermore, the local density approximation (LDA) was used for the adsorption energy calculations of Al and Cr atoms on the HfC (0 0 1) surface. For Al, Cr atoms on the surface, the adsorption energies confirm the preference for adsorption at the on-top C atoms sites. Moreover, compared with Al atoms, Cr atoms are more easily adsorbed on the HfC (0 0 1) surface. The Cr adatom–surface bonds are stronger than the Al adatom–surface bonds. The C–Al bonds are ionic while the C–Cr bonds present the covalent and metallic characteristics in nature.

How Are Non-Bonded G···Z (Z = O, S, and Se) Distances at Benzene 1,2-, Naphthalene 1,8-, and Anthracene 1,8,9-Positions Controlled? An Approach to Causality in Weak Interactions

Abstract Mechanisms operating in weak interactions are sometimes quite different from those in strong ones. Factors to control the non-bonded G···Z (Z = O, S, and Se) distances at naphthalene 1,8-positions in 8-G-1-RZC10H6 are analyzed as an example to clarify the causality of weak interactions. Structures around Z in 8-G-1-RZC10H6 are well explained by three types, A, B, and C, where the Z–CR bond is perpendicular to the naphthyl plane in A, it is placed on the plane in B, and C is intermediate between A and B. The r(Z···Z) distances in 1,8-(RZ)2C10H6 are observed to increase in an order of BB < CC < AB < AA. It seems difficult to explain the sequence based on the magnitudes of the closed-shell Z···Z interactions, since the BB interaction must not be so strong especially for Z of heavier atoms. The mechanism to control the r(Z···Z) distances is demonstrated to be the reflection from the repulsive interactions between R and H at the 2-position in 1,8-(RZ)2C10H6. Similar phenomena are also detected in the observed and calculated results at the benzene 1,2-, anthracene 1,8,9-, and anthraquinone 1,8,9-positions, which are explained based on the mechanism. Proposed idea and mechanism will help us to understand what happens in such weak interactions.

Redox Modification of EMACs Through the Tuning of Ligands: Heptametal(II) Complexes of Pyrazine‐Modulated Oligo‐α‐pyridylamido Ligands

AbstractUsing pyrazine‐modulated oligo‐α‐pyridylamido ligands N2‐(pyrazin‐2‐yl)‐N6‐[6‐(pyrazin‐2‐ylamino)pyridin‐2‐yl]pyridine‐2,6‐diamine (H3pzpz) and N2‐(pyrazin‐2‐yl)‐N6‐[6‐(pyridin‐2‐ylamino)pyridin‐2‐yl]pyridine‐2,6‐diamine (H3tpz), linear heptametal(II) extended metal atom chains (EMACs) [M7(μ7‐L)4X2] [L = pzpz3–, M = NiII, X = Cl– (1), NCS– (2); M = CrII, X = Cl– (3), NCS– (4); L = tpz3–, M = CrII, X = Cl– (5), NCS– (6)] were synthesized and structurally characterized. Electrochemical studies showed that heptanickel(II) complexes can undergo one reversible oxidation at +0.46 V for 1 and +0.52 V for 2. Chromium(II) species 3 exhibited two reversible, one‐electron oxidation peaks at +0.61 and +0.88 V, and 5 exhibited three reversible, one‐electron oxidation peaks at +0.40, +0.68 and +1.07 V. The redox peaks shifted positively when axial ligands changed from chloride to thiocyanate anions, at +0.67 and +0.92 V for 4 and +0.44, +0.73 and +1.11 V for 6. The introduction of electron‐withdrawing pyrazine rings to the spacer ligand retarded oxidation of the heptametal EMACs and stabilized the complexes. In nickel(II) species 1 and 2, both terminal nickel atoms exist in spin state S = 1 whereas all the inner nickel atoms exist in spin state S = 0. Temperature‐dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about–4 cm–1. Chromium(II) species 3–6 showed a localized structure consisting of three quadruple Cr–Cr bonds and a single terminal CrII atom. Magnetic study revealed a quintet ground state resulting from the isolated, high‐spin CrII atom.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

BS DFT and BS HDFT studies of CrCr sextuple bond from the viewpoint of electron correlation effects

AbstractThe nature of CrCr bond and effectiveness of broken‐symmetry (BS) density functional theory (DFT) and hybrid DFT (HDFT) methods are examined in terms of electron correlation effects. Two types of correlation energies, i.e., static and dynamical correlation energies are estimated by relative energy curves and correlation energy curves. Energy decomposition analyses and natural orbital analyses reveal how these electron correlation effects affect the CrCr bonding. In addition, the effect of those effects to an energy gap between the lowest spin (LS) state (S = 0) and the highest spin (HS) state (S = 6) is also discussed with respect to calculations of effective exchange integrals (J). © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Analysing the chromium–chromium multiple bonds using multiconfigurational quantum chemistry

This Letter discusses the nature of the chemical bond between two chromium atoms in different di-chromium complexes with the metal atoms in different oxidation states. Starting with the Cr diatom, with its formally sextuple bond and oxidation number zero, we proceed to analyse the bonding in some Cr(I)–Cr(I) XCrCrX complexes with X varying from F, to Phenyl, and Aryl. The bond distance in these complexes varies over a large range: 1.65–1.83 Å and we suggest explanations for these variations. A number of di-chromium complexes with bond distances around or shorter than 1.80 Å have recently been synthesized and we study one of these complexes, Cr 2(diazadiene) 2 and show how the Cr–Cr bond order is related to the oxidation number and the ligand bonding, factors that are all involved in the determination of the short Cr–Cr bond length: 1.80 Å. The discussion is based on the use of multiconfigurational wave functions, which give a qualitatively correct description of the electronic structure in these multiply bonded systems.

Chromium (iii)-bis(iminophosphoranyl)methanido complexes: synthesis, X-ray crystal structures and catalytic ethylene oligomerization

Bis(aminophoshonium) salts derived from bis(diphenylphosphino)methane were prepared and triply deprotonated with potassium hexamethyldisilazane to yield the corresponding bis(iminophosphoranyl)methanides, which were then subjected to coordination with CrIIICl3(THF)3. Binuclear complexes of type [(HC(PPh2NR)2Cr(μ-Cl)(Cl))2] (R = iPr, tBu) with an octahedral coordination geometry and a long C–Cr bond were obtained and structurally characterized. In the case of R = o-MeO-C6H4, a monomeric species featuring no C–Cr bond is observed. Preliminary evaluation of catalytic activities of these complexes in ethylene oligomerization and polymerization is reported.

New Mixed‐Metal Carbonyl Complexes Containing Bridging 2‐Mercapto‐1‐methylimidazole Ligand

AbstractReaction of [Mn2(CO)10] with 2‐mercapto‐1‐methylimidazole in the presence of Me3NO at 25 °C afforded two new dimanganese complexes [Mn2(CO)6(μ‐SN2C4H5)2] (1) and [Mn2(CO)7(μ‐SN2C4H5)2] (2). Compound 1 consists of two μ‐SN2C4H5 ligands, each bound through the sulfur atom to two Mn atoms and through the nitrogen atom to one Mn atom forming a four‐membered chelate ring. Compound 2 was found to consist of one μ‐SN2C4H5 ligand in a similar bonding mode to 1 but another μ‐SN2C4H5 ligand coordinates through the sulfur atom to one Mn atom and through the nitrogen atom to another Mn atom. Compound 1 was also obtained as the only product from the reaction of [Mn2(CO)8(NCMe)2] with 2‐mercapto‐1‐methylimidazole. In contrast, a similar reaction of [Re2(CO)8(NCMe)2] with 2‐mercapto‐1‐methylimidazole led to the formation of the di‐, tri‐, and tetranuclear complexes [Re3(CO)8(μ‐CO)(μ3‐SN2C4H5)2(μ‐H)] (3), [Re4(CO)12(μ‐SN2C4H5)4] (4), and [Re2(CO)6(μ‐SN2C4H5)2] (5). Compound 3 provides a unique example of a hydrido trirhenium compound. The reaction of [Cr(CO)3(NCMe)3] and [Mo(CO)3(NCMe)3] with 1 in refluxing THF afforded the mixed metal complexes [CrMn2(CO)8(μ‐CO)2(μ3‐SN2C4H5)2] (6) and [MoMn2(CO)8(μ‐CO)2(μ3‐SN2C4H5)2] (7), respectively, in which two Mn–M (M = Mo, Cr) bonds were formed. In contrast, a similar treatment of [W(CO)3(NCMe)3] with 1 yielded two W‐Mn complexes [Mn2W(CO)8(μ‐CO)2(μ3‐SN2C4H5)2] (8) and [Mn2W(CO)7(μ‐CO)2(SN2C4H5)(μ3‐SN2C4H5)2] (9). Treatment of 1 with [Fe3(CO)12] at 70‐75 °C afforded the trinuclear mixed‐metal complex [FeMn2(CO)8(μ‐CO)(μ3‐SN2C4H5)2] (10) and the diiron side product [Fe2(CO)6(μ‐S2N2C4H5)2] (11). Compounds 6‐10 have a bent open structure of three metal atoms linked by two metal‐metal bonds and all, except 9 and 10, contain a noncrystallographic two‐fold axis of symmetry. Compound 9 is structurally similar to 8, but it contains a SN2C4H6 ligand, mono coordinated through the exocyclic sulfur atom to the W atom and a Mn–Mn bond instead of a Mn–W bond. Compound 11 comprises two bridging S2N2C4H5 ligands, which arise from the coupling of 2‐mercapto‐1‐methylimidazole with sulfur.

Bond Length and Bond Order in One of the Shortest Cr−Cr Bonds

Multiconfigurational quantum chemical calculations on the R-diimines dichromium compound confirm that the Cr-Cr bond, 1.80 A, is among the shortest Cr(I)-Cr(I) bonds. However, the bond between the two Cr atoms is only a quadruple bond rather than a quintuple bond. The reason why the bond is so short has to be attributed to the strain in the NCCN ligand moieties.

77Se NMR Chemical Shifts of 9-(Arylselanyl)triptycenes: New Standard for Planar Structures of ArSeR and Applications to Determine the Structures in Solutions

A set of new delta(Se) parameters is proposed as a standard for the planar (pl) orientational effect of p-YC(6)H(4) (Ar) in ArSeR, employing 9-(arylselanyl)triptycenes (1: p-YC(6)H(4)SeTpc). The Se-C(R) bond in ArSeR is placed on the Ar plane in pl and it is perpendicular to the plane in pd. Large upfield shifts are observed for Y = NMe(2), OMe, and Me (-22 to -6 ppm) and large downfield shifts for Y = COOEt, CN, and NO(2) (19-37 ppm), relative to Y = H, with small upfield and moderate downfield shifts by Y of halogens (-1 ppm for Y = F and 4 ppm for Y = Cl and Br). This must be the result of the p(Se)-pi(C(6)H(4))-p(Y) conjugation in 1 (pl). While the character of delta(Se) in 1 (pl) is very similar to that in 9-(arylselanyl)anthracenes (2 (pl)), it is very different from that of 1-(arylselanyl)anthraquinones (3 (pd)). Sets of delta(Se) of 1 and 2 must serve as the standard for pl and that of 3 does for pd in solutions. Structures of various ArSeR in solutions are determined from the viewpoint of the orientational effect based on the standard delta(Se) of 1-3. While the structure of 2-methyl-1-(arylselanyl)naphthalenes is concluded to be all pl in solutions, those of 8-chloro- and 8-bromo-1-(arylselanyl)naphthalenes are all pd, except for Y = COOEt, CN, and NO(2): The equilibrium between pd and pl contributes to those with Y = COOEt, CN, and NO(2). The structure of 1-(arylselanyl)naphthalenes changes depending on Y. The structures of ArSeMe and ArSeCOPh are shown to be pl and pd, respectively, in solutions. Those of ArSePh and ArSeAr seem to change depending on Y. delta(Se) of 1-3 are demonstrated to serve as the standard to determine the structures in solutions. The rules of thumb derived from the characters in delta(Se) for 1-3 are very useful to determine the structures of ArSeR in solutions, in addition to the analysis based on the plots.

Remarkably Short Metal–Metal Bonds: A Lantern‐Type Quintuply Bonded Dichromium(I) Complex

The field of quadruply bonded dinuclear complexes in which two metal atoms are embraced by eight ligands has been considered mature. The bonding and electronic structures of these compounds have been well understood, ever since the discovery of the first dimetal species containing a quadruple bond, [Re2Cl8] 2 , over 40 years ago. The quest for thermally stable and isolable dinuclear complexes with higher bond orders is one aim of chemists in this field. From the synthetic point of view, quintuply bonded dinuclear complexes have become the focus in the past few years. On the theoretical side, the hypothetical RMMRmolecules (M= Cr, Mo, W, U; R=H, F, Cl, Br, CN, Me) have been highlighted, particularly the trans-bent structures. These feature a quintuple bond between two metal atoms, and for RCrCrR, the calculated Cr–Cr bond lengths are in the range of 1.64–1.78 4. Experimentally, a recent landmark advance was the isolation of a quintuply bonded dichromium(I) complex supported by two monodentate bulky carbyl ligands [Ar’CrCrAr’] (Ar’=C6H3-2,6-(C6H3-2,6iPr2)2), [12] in which, coincidentally, a trans-bent (C2h) geometry is adopted and the two chromium atoms share five electron pairs in five bonding molecular orbitals according to computational analyses. 14] Moreover, D2h-symmetric [Cr2(m-h L)2] ( L=N,N’-bis(2,6-diisopropylphenyl)-1,4-diazadiene) was recently reported to feature a very short Cr–Cr distance of 1.8028(9) 4 and calculated to display some degree of fivefold bonding. We have been interested in the pursuit of low-coordinate and multiply bonded dinuclear complexes since our first report on the unconventional quadruply bonded dimolybdenum complex [Mo2{m-h -(DippN)2SiMe2}2] (Dipp= 2,6iPrC6H3), in which each Mo atom is ligated by only two nitrogen atoms. Accordingly, we were interested in the preparation of multiply bonded dinuclear complexes supported by ancillary ligands which can minimize the metal– ligand p-bonding interaction and maximize the metal–metal interaction. Inspired by the hypothetical eclipsing molecule M2L6, proposed by Hoffmann et al., [17] in which M M could be a quintuple bond, we set out to explore the possibility of synthesizing such complexes. Here we report the use of amidinate ligand ArNC(H)NAr (Ar= 2,6-C6H3(CH3)2) to stabilize mixed-valent Cr2 3+ complex [Cr2(ArNC(H)NAr)3] (2) with formal Cr Cr bond order of 4.5 and its one-electron reduced Cr2 2+ species [Cr2(ArNC(H)NAr)3] (3 with formal Cr Cr bond order of 5.0). Both of these species have very short Cr Cr bonds; the bonding in these two compounds was studied theoretically. Reduction of dichromium bis(amidinate) dichlorido complex [{Cr(thf)}2(m-Cl)2{m-h -(ArNC(H)NAr)}2] (1), [18] in which the Cr–Cr distance is 2.612(1) 4, with KC8 gave redbrown mixed-valent dichromium tris(amidinate) compound 2 in 47% yield (Scheme 1). Compound 2 is paramagnetic, as

1,4-Bis(chloromethyl)naphthalene

In the title mol­ecule, C12H10Cl2, the torsion angles Cr—Cr—Cm—Cl around the Cm—Cr bonds have values of −104.1 (4) and −101.9 (4)°, where Cm is a methylene and Cr is a ring C atom. The mol­ecules related by translation along the b axis are arranged into stacks by π–π inter­actions between unsubstituted and substituted aromatic rings of the naphthalene ring system (centroid–centroid distance = 3.940 Å).

Formal chromium–chromium triple bonds and bent rings in the binuclear cycloheptatrienylchromium carbonyls (C 7H 7) 2Cr 2(CO) n ( n = 6, 5, 4, 3, 2, 1, 0): A density functional theory study

Binuclear cycloheptatrienylchromium carbonyls of the type (C 7H 7) 2Cr 2(CO) n ( n = 6, 5, 4, 3, 2, 1, 0) have been investigated by density functional theory. Energetically competitive structures with fully bonded heptahapto η 7-C 7H 7 rings are not found for (C 7H 7) 2Cr 2(CO) n structures having two or more carbonyl groups. This result stands in contrast to the related (C n H n ) 2M 2(CO) n (M = Mn, n = 6; M = Fe, n = 5; M = Co, n = 4) systems. Most of the predicted (C 7H 7) 2Cr 2(CO) n structures have bent trihapto or pentahapto C 7H 7 rings and Cr Cr distances in the range 2.4–2.5 Å suggesting formal triple bonds. In some cases rearrangement of the heptagonal C 7H 7 ring to a tridentate cyclopropyldivinyl or tridentate bis(carbene)alkyl ligand is observed. In addition structures with CO insertion into the C 7H 7–Cr bond are predicted for (C 7H 7) 2Cr 2(CO) n ( n = 6, 4, 2). The global minima found for the (C 7H 7) 2Cr 2(CO) n derivatives for n = 6, 5, and 4 are (η 5-C 7H 7)(OC) 2Cr Cr(CO) 4(η 1-C 7H 7), (η 3-C 7H 7)(OC) 2Cr Cr(CO) 3(η 2,1- C 7H 7), and (η 5-C 7H 7) 2Cr 2(CO) 4, respectively. The global minima for (C 7H 7) 2Cr 2(CO) n ( n = 3, 2) have rearranged C 7H 7 groups. Singlet and triplet structures with heptahapto η 7-C 7H 7 rings are found for the dimetallocenes (η 7-C 7H 7) 2Cr 2(CO) and (η 7-C 7H 7) 2Cr 2, with the singlet structures being of much lower energies in both cases.

First-principles investigations of disorder effects on electronic structure and magnetic properties inSr2CrMoO6

The electronic structures and magnetic properties are reported for ordered and disorderedSr2CrMoO6 presenting oxygen vacancies or/and antisite defects (ASs). We investigate the stability ofan antiparallel (AP) magnetic moment on Cr antisites and the calculations show that thesesolutions are more stable relative to the parallel solution for AS defects with or withoutoxygen vacancies. Electronic band structure calculations indicate that the perfectSr2CrMoO6 is half-metallic, and the half-metallic character is preserved forSr2CrMoO6 containing only oxygen vacancies, while the half-metallic nature is destroyed when25% ASs (50% ASs) with or without oxygen vacancies is present. For 25% ASswith two oxygen vacancies, the system possibly shows nonmetallic behavior. Theexperimentally observed reduction of the magnetic moment mainly arises from anantiferromagnetic coupling of Cr–O–Cr (Cr–Cr) bonds in a disordered sample.

Pressure dependence of color of natural uvarovite: the barochromic effect

The electronic absorption spectra of natural uvarovite containing 62 mole% of the Cr3+ end-member were studied at pressures between 10−4 and ca. 13 GPa using DAC techniques combined with microscope spectrometric device. With increasing pressure, a barochromic effect with change from green to red color of the garnet specimen was observed. This change could be interpreted on the basis of the spectra and the data points derived in an ICE color card. The evaluation of crystal field data from the spectra showed that 10Dq of chromium increases on pressure while the Racah parameter B, and thus the nature of the chemical bond of Cr–O does not change significantly.

The Shortest Metal−Metal Bond Yet: Molecular and Electronic Structure of a Dinuclear Chromium Diazadiene Complex

Reduction of a diazadiene chromium halide complex, [(HLiPr)Cr(μ-Cl)]2 (1, HLiPr = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (HLiPr)2Cr2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr2 unit spanned by two diazadiene ligands. The very short Cr−Cr distance (1.8028(9) A) and low formal oxidation state of the Cr atoms suggest that the Cr−Cr bond order may be greater than 4. Spin-restricted and spin-unrestricted DFT calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal−metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr−Cr bond.

Spectroscopic Detection and Theoretical Confirmation of the Role of Cr2(CO)5(C5R5)2 and ·Cr(CO)2(ketene)(C5R5) as Intermediates in Carbonylation of NNCHSiMe3 to OCCHSiMe3 by ·Cr(CO)3(C5R5) (R = H, CH3)

Conversion of N=N=CHSiMe3 to O=C=CHSiMe3 by the radical complexes .Cr(CO)3C5R5 (R = H, CH3) derived from dissociation of [Cr(CO)3(C5R5)]2 have been investigated under CO, Ar, and N2 atmospheres. Under an Ar or N2 atmosphere the reaction is stoichiometric and produces the Cr[triple bond]Cr triply bonded complex [Cr(CO)2(C5R5)]2. Under a CO atmosphere regeneration of [Cr(CO)3(C5R5)]2 (R = H, CH3) occurs competitively and conversion of diazo to ketene occurs catalytically as well as stoichiometrically. Two key intermediates in the reaction, .Cr(CO)2(ketene)(C5R5) and Cr2(CO)5(C5R5)2 have been detected spectroscopically. The complex .Cr(13CO)2(O=13C=CHSiMe3)(C5Me5) has been studied by electron spin resonance spectroscopy in toluene solution: g(iso) = 2.007; A(53Cr) = 125 MHz; A(13CO) = 22.5 MHz; A(O=13C=CHSiMe3) = 12.0 MHz. The complex Cr2(CO)5(C5H5)2, generated in situ, does not show a signal in its 1H NMR and reacts relatively slowly with CO. It is proposed to be a ground-state triplet in keeping with predictions based on high level density functional theory (DFT) studies. Computed vibrational frequencies are also in good agreement with experimental data. The rates of CO loss from 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)2(C5H5)]2 and CO addition to 3Cr2(CO)5(C5H5)2 producing 1[Cr(CO)3(C5H5)]2 have been measured by kinetics and show DeltaH approximately equal 23 kcal mol(-1) for both processes. Enthalpies of reduction by Na/Hg under CO atmosphere of [Cr(CO)n(C5H5)]2 (n = 2,3) have been measured by solution calorimetry and provide data for estimation of the Cr[triple bond]Cr bond strength in [Cr(CO)2(C5H5)]2 as 72 kcal mol(-1). The complex [Cr(CO)2(C5H5)]2 does not readily undergo 13CO exchange at room temperature or 50 degrees C implying that 3Cr2(CO)5(C5H5)2 is not readily accessed from the thermodynamically stable complex [Cr(CO)2(C5H5)]2. A detailed mechanism for metalloradical based conversion of diazo and CO to ketene and N2 is proposed on the basis of a combination of experimental and theoretical data.

Ferromagnetism vs. Antiferromagnetism of the Dimorphic HoCrO4 Oxide.

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Precursors for Assembly of Supramolecules Containing Quadruply Bonded Cr24+ Units: Systematic Preparation of Cr2(formamidinate)n(acetate)4–n (n = 2–4)

AbstractFour quadruply bonded dichromium complexes with mixed‐ligand sets, Cr2(formamidinate)n(acetate)4–n (n = 2–4), were synthesized from reactions of anhydrous Cr2(O2CCH3)4 and formamidinate anions. The Cr–Cr bond lengths fall in the range for “supershort” Cr–Cr bonds as they vary from 1.8897(5) Å to 2.012(1) Å. The distance variation depends on the presence or absence of weak axial interactions. Because formamidinate ligands are less labile than acetate groups, these compounds may be useful building blocks for the construction of neutral supramolecules having dichromium units linked by polydentate dianions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Substituent effect in para substituted Cr(CO) 5–pyridine complexes

Systematic studies on the substituent effect in para substituted Cr(CO) 5–pyridine complexes have been carried out on the basis of DFT quantum-chemical calculations. Ten simple and mostly common substituents were chosen in order to analyze possibly the largest spectrum of substituent effects. The following substituents were taken into consideration: NO, NO 2, CN, CHO, F, H, CH 3, OCH 3, OH and NH 2. Additionally, the Cr–N and Cr–C bonds were characterized on the basis of Atoms in Molecules topological analysis of electron density. It has been found that the substituents in position 4 of the pyridine ring influence the Cr–N bond of Cr(CO) 5–pyridine complex in a systematic manner, as a result of with, the pyridine moiety has a diversified ability of participating in the interaction with the Cr atom of Cr(CO) 5 moiety. It has also been found, that the electron withdrawing substituents additionally stabilize the Cr–N bond, whereas the electron donating ones weaken it. The substituent effect mainly affects the π-component of the Cr–N bond. This effect proceeds in the whole Cr–pyridine–R moiety, and it is additionally reflected in the corresponding changes in metal–carbonyl bonds, particularly the trans Cr–CO bond.

Monomeric, Two-Coordinate, Univalent Chromium(I) Compounds: Steric Prevention of Metal−Metal Bond Formation

Monomeric, two-coordinate, univalent, aryl chromium(I) species [(3,5-iPr2−Ar*)Cr(L)] (L = THF, PMe3) have been synthesized with use of a very bulky terphenyl ligand which prevents Cr−Cr bond formation.