Frenking and Frohlich discuss the progress that has been made mainly in the past decade toward an understanding of the binding interactions in transition-metal compounds.
The nature of the chemical bond in mixed carbene−halogen complexes (1)TMX (X = F−I) and bis(carbene) complexes (1)2TM+ of group 11 metals (TM = Cu, Ag, Au) with imidazol-1-ylidene (1) as ligand has ...been investigated at the BP86 level of theory using an energy decomposition analysis (EDA). The metal−carbene bonds are mainly held together by classical electrostatic attraction, which contributes >65% of the binding interactions. The metal−carbene bonds are very strong. In the bis(carbene) complexes, the N-heterocyclic carbene ligand 1 is bonded even more strongly than in the mixed carbene−halogen complexes. In the bis(carbene) complexes, orbital interactions are slightly more important than in the mixed carbene−halogen complexes but the covalent contribution is always <35% of the total attractive interaction. The orbital interaction part of the bonding has only ∼20% π back-bonding. The calculated data are not very different from previous EDA results for the Fischer carbene complex (CO)5W−C(OH)2. The EDA results suggest that R2C←ML n π back-donation in complexes with N-heterocyclic carbenes is not substantially smaller than in classical Fischer carbene complexes bearing two π donor groups R.
The results of an energy partitioning analysis of three classes of transition metal complexes are discussed. They are (i) neutral and charged isoelectronic hexacarbonyls TM(CO)
6
q
(TM
q
=Hf
2−, Ta
...−, W, Re
+, Os
2+, Ir
3+); (ii) Group-13 diyl complexes (CO)
4FeER (E=B, Al, Ga, In, Tl; R=Cp, Ph), Fe(ECH
3)
5 and Ni(ECH
3)
4; (iii) complexes with cyclic π-donor ligands Fe(Cp)
2 and Fe(η
5-N
5)
2. The results show that Dewar's molecular orbital model can be recovered and that the orbital interactions can become quantitatively expressed by accurate quantum chemical calculations. However, the energy analysis goes beyond the MO model and gives a much deeper insight into the nature of the metal–ligand bonding. It addresses also the question of ionic versus covalent bonding as well as the relative importance of σ and π bonding contributions.
The bonding model suggested for metal–olefin complexes, which was suggested by Dewar 50 years ago, is found by an energy partitioning analysis to be a valid description of the bonding in transition metal complexes with ligands CO, Group-13 diyl species ER (E=B, Al, Ga, In, Tl; R=Cp, Ph, CH
3) and with cyclic π-donor ligands Cp and cyc-N
5 in Fe(Cp)
2 and Fe (η
5-N
5)
2.
The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using ...density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a 3+2 mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO4 again prefers a 3+2 pathway across the CC moiety whereas, for the rhenium oxides LReO3, the 2+2 barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO4 - and (H3PN)ReO3 add across the CO bond while MeReO3 and CpReO3 favor the addition across the CC moiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting η5, η3, and η1 bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor−acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.
The intrinsic strength of π interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction ...energy between the conjugating fragments. The results of the EDA of the trans‐polyenes H2CCH(HCCH)nCHCH2 (n=1–3) show that the strength of π conjugation for each CC moiety is higher than in trans‐1,3‐butadiene. The absolute values for the conjugation between SiSi π bonds are around two‐thirds of the conjugation between CC bonds but the relative contributions of ΔEπ to ΔEorb in the all‐silicon systems are higher than in the carbon compounds. The π conjugation between CC and CO or CNH bonds in H2CCHC(H)O and H2CCHC(H)NH is comparable to the strength of the conjugation between CC bonds. The π conjugation in H2CCHC(R)O decreases when R=Me, OH, and NH2 while it increases when R=halogen. The hyperconjugation in ethane is around a quarter as strong as the π conjugation in ethyne. Very strong hyperconjugation is found in the central CC bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X3CCY3 (X,Y=Me, SiH3, F, Cl) is stronger than in the parent compound particularly when X,Y=SiH3 and Cl. The hyperconjugation in donor–acceptor‐substituted ethanes may be very strong; the largest ΔEπ value was calculated for (SiH3)3CCCl3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X3CCY3 shows that donation of the donor‐substituted moiety to the acceptor group is as expected the most important contribution but the reverse interaction is not negligible. The relative strengths of the π interactions between two CC double bonds, one CC double bond and CH3 or CMe3 substituents, and between two CH3 or CMe3 groups, which are separated by one CC single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HCCC(SiH3)3 and HCCCCl3. The extra stabilization of alkenes and alkynes with central multiple bonds over their terminal isomers coming from hyperconjugation is bigger than the total energy difference between the isomeric species. The hyperconjugation in MeC(R)O is half as strong as the conjugation in H2CCHC(R)O and shows the same trend for different substituents R. Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of σ interactions and electrostatic forces may compensate for the hyperconjugative effect.
The strength of π conjugation and hyperconjugation between various types of π bonds has been calculated for 62 molecules using energy decomposition analysis (EDA). EDA suggests that there is strong hyperconjugation in cubylcubane and tetrahedranyltetrahedrane that may contribute to the very short central CC bonds (see picture). However, strong hyperconjugation may also be found in long bonds. The strongest hyperconjugation is calculated for Cl3CC(SiH3)3, which has a rather long CC bond.
Summary
Glycosaminoglycans are accumulated in both mucopolysaccharidoses (MPS) and mucolipidoses (ML). MPS I, II, III and VII and ML II and ML III patients cannot properly degrade heparan sulphate ...(HS). In spite of the importance of HS storage in the metabolic pathway in these diseases, blood and urine HS levels have not been determined systematically using a simple and economical method. Using a new ELISA method using anti‐HS antibodies, HS concentrations in blood and urine were determined in MPS and ML II and ML III patients. HS concentrations were determined in 156 plasma samples from MPS I (n = 23), MPS II (n = 26), MPS III (n = 24), MPS IV (n = 62), MPS VI (n = 5), MPS VII (n = 5), ML II (n = 8) and ML III (n = 3), and 205 urine samples from MPS I (n = 33), MPS II (n = 33), MPS III (n = 30), MPS IV (n = 82), MPS VI (n = 7), MPS VII (n = 9), ML II (n = 8) and ML III (n = 3). The ELISA method used monoclonal antibodies against HS. MPS I, II, III and VII and ML II and III patients had significant elevation in plasma HS, compared to the age‐matched controls (p < 0.0001). Eighty‐three out of 89 (93.3%) of individual values in the above MPS types and ML were above the mean +2SD of the controls. In urine samples, 75% of individual values in patients with those types were above the mean +2SD of the controls. In contrast to the previous understanding of the HS metabolic pathway, plasma HS levels in all five MPS VI and 15% of MPS IV patients were elevated above the mean +2SD of the controls. These findings suggest that HS concentration determined by ELISA, especially in plasma, could be a helpful marker for detection of the most severe MPS I, II, III, VI and VII and ML II, distinguishing them from normal populations.
DFT calculations at BP86/QZ4P have been carried out for different structures of E2H2 (E = C, Si, Ge, Sn, Pb) with the goal to explain the unusual equilibrium geometries of the heavier group 14 ...homologues where E = Si−Pb. The global energy minima of the latter molecules have a nonplanar doubly bridged structure A followed by the singly bridged planar form B, the vinylidene-type structure C, and the trans-bent isomer D1. The energetically high-lying trans-bent structure D2 possessing an electron sextet at E and the linear form HE⋮EH, which are not minima on the PES, have also been studied. The unusual structures of E2H2 (E = Si−Pb) are explained with the interactions between the EH moieties in the (X2Π) electronic ground state which differ from C2H2, which is bound through interactions between CH in the a4Σ- excited state. Bonding between two (X2Π) fragments of the heavier EH hydrides is favored over the bonding in the a4Σ- excited state because the X2Π → a4Σ- excitation energy of EH (E = Si−Pb) is significantly higher than for CH. The doubly bridged structure A of E2H2 has three bonding orbital contributions: one σ bond and two E−H donor−acceptor bonds. The singly bridged isomer B also has three bonding orbital contributions: one π bond, one E−H donor−acceptor bond, and one lone-pair donor−acceptor bond. The trans-bent form D1 has one π bond and two lone-pair donor−acceptor bonds, while D2 has only one σ bond. The strength of the stabilizing orbital contributions has been estimated with an energy decomposition analysis, which also gives the bonding contributions of the quasi-classical electrostatic interactions.
The nature of the chemical bonding in Fischer- and Schrock-type carbene complexes as well as in complexes with N,N-heterocyclic carbene ligands has been analyzed with charge- and energy-partitioning ...methods.
In this work, we summarize recent theoretical studies of our groups in which modern quantum chemical methods are used to gain insight into the nature of metal–ligand interactions in Fischer- and Schrock-type carbene complexes. It is shown that with the help of charge- and energy-partitioning techniques it is possible to build a bridge between heuristic bonding models and the physical mechanism which leads to a chemical bond. Questions about the bonding situation which in the past were often controversially discussed because of vaguely defined concepts may be addressed in terms of well defined quantum chemical expressions. The results of the partitioning analyses show that Fischer and Schrock carbenes exhibit different bonding situations, which are clearly revealed by the calculated data. The contributions of the electrostatic and the orbital interaction are estimated and the strength of the σ donor and π acceptor bonding in Fischer complexes are discussed. We also discuss the bonding situation in complexes with N,N-heterocyclic carbene ligands.
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![[2+2] versus [3+2] Addition...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0002-7863/small "[2+2] versus [3+2] Addition of Metal Oxides Across CC Double Bonds: Toward an Understanding of the Surprising Chemo- and Periselectivity of Transition-Metal-Oxide Additions to Ketene")
