Interactions in dimers of model alkali metal derivatives M(2)X(2) (M = Li or Na or K; X = H or F, Cl, OH) are studied in the frame of the quantum theory of atoms in molecules (QTAIM) using the interacting quantum atoms approach (IQA). Contrary to opinion prevalent in QTAIM studies, the interaction between two anions linked by a bond path is demonstrated to be strongly repulsive. One may therefore say that a bond path does not necessarily indicate bonding interactions. The interactions between two anions or two cations that are not linked by a bond path are also strongly repulsive. The repulsive anion-anion and cation-cation interactions are outweighed by much stronger attractive anion-cation interactions, and the model molecules are therefore in a stable state. The attractive Ehrenfest forces (calculated in the frame of the QTAIM) acting across interatomic surfaces shared by anions in the dimers do not reflect the repulsive interactions between anions. Probable reasons of this disagreement are discussed. The force exerted on the nucleus and the electrons of a particular atom by the nucleus and the electrons of any another atom in question is proposed. It is assumed that this force unambiguously exposes whether basins of two atoms are attracted or repelled by each other in a polyatomic molecule.

Calculations within the framework of the interacting quantum atoms (IQA) approach have shown that the interactions of the helium atom with both tertiary, tC, and secondary, sC, carbon atoms in the metastable He@adamantane (He@adam) endohedral complex are bonding in nature, whereas the earlier study performed within the framework of Bader's quantum theory of atoms in molecules (QTAIM) revealed that only He---tC interactions are bonding. The He---tC and He---sC bonding interactions are shown to be forced by the high pressure that the helium and carbon atoms exert upon each other in He@adam. The occurrence of a bonding interaction between the helium and sC atoms, which are not linked by a bond path, clearly shows that the lack of a bond path between two atoms does not necessarily indicate the lack of a bonding interaction, as is asserted by QTAIM. IQA calculations showed that not only the destabilization of the adamantane cage, but also a huge internal destabilization of the helium atom, contribute to the metastability of He@adam, these contributions being roughly equal. This result disproves previous opinions based on QTAIM analysis that only the destabilization of the adamantane cage accounts for the endothermicity of He@adam. Also, it was found that there is no homeomorphism of the ρ(r) and -v(r) fields of He@adam. Comparison of the IQA and QTAIM results on the interactions in He@adam exposes other deficiencies of the QTAIM approach. The reasons for the deficiencies in the QTAIM approach are analyzed.

and C2-C3 as well as of the difference AC-C of these two bond lengths with the corresponding values of theoretical models of the propargyl and allenyl type. This leads also to a correction of a n earlier assigned structural type.Organometallic derivatives of propynes and allenes are widely used in the synthesis of acetylenes and aliened']. In contrast, the structures of such compounds -we restrict ourselves in this context to alkali metal derivativesL2] -are not as well-known as their reactions, although structural details might contribute to a better understanding of the ambident nature of these nucleophiles. The structures of [lLi . THFI2, [2-Li . OEt2I2, and [3-LiI2 have first been described by the Schleyer However, experimental details of these X-ray crystal structure investigations have not been published to date. A brief summary of some data of these species together with bond lengths and angles of [lNa . 2 TMEDA]L41 are given in Table 1.The most recent calculations of the model allenyl and propargyl structures 4-, 5-(CJ, 4-Li, [4-LiI2, and [5-LiI2 have led to the results listed in Table 2[5aI.The results of Table 2 can be summarized as follows: 1. The slightly bent allenyl anion 4-(Cl-C2-C3 = 172.8') has different bond lengths: 128.0 versus 136.6 pm; AC-C = 8.6 pm.2. The propargyl anion structure 5-(C,) is not a minimum; the bond lengths of the almost linear 5-(C,) (Cl-C2-C3 = 177.0') are significantly more different than in the case of 4-: 124.8 and 139.7 pm; AC-C = 14.9 Pm.3. In allenyllithium 4-Li, the bond lengths correspond to those in 4-: 127.6 and 137.3 pm; AC-C = 9.7 pm. The Cl-C2-C3 angle is much smaller (156.5") than in 4-(172.8') due to the q3-bonding of Li.4. If dimerization of allenyllithium 4-Li occurs along Cl -Li, the allyllithium dimer [4-LiJ2 results with even more equal bond lengths (129.6 and 134.5 pm; AC-C 4.9 pm) than in the monomer 4-Li. In neutral allene the CC bonds the Me't-Bu 13.5 1 7 2 . 9 ( 9 )are 131 pm longL6]. A second Li at C1 thus promotes polarization towards and charge localization at this carbon atom

Multireference [complete active space self-consistent field (CASSCF) and multiconfigurational quasidegenerate perturbation theory (MCQDPT)] and single-reference ab initio (Moller-Plesset second order perturbation theory (MP2) and coupled clusters with singles, doubles and noniterative triples [CCSD(T)]) and density functional theory (PBE and B3LYP) electronic structure calculations of V(C(6)H(6))(+) half-sandwich in the states of different multiplicities are described and compared. Detailed analyses of the geometries and electronic structures of the all found states are given; adiabatic and diabatic dissociation energies are estimated. The lowest electronic state of V(C(6)H(6))(+) half-sandwich was found to be the quintet (5)B(2) state with a slightly deformed upside-down-boat-shaped benzene ring and d(4) configuration of V atom, followed by a triplet (3)A(2) state lying about 4 kcal/mol above. The lowest singlet state (1)A(1)(d(4)) lies much ( approximately 28 kcal/mol) higher. MCQDPT calculated adiabatic dissociation energy (53.6 kcal/mol) for the lowest (5)B(2)(d(4)) state agrees well with the current 56.4 (54.4) kcal/mol experimental estimate, giving a preference to the lower one. Compared to MCQDPT, B3LYP hybrid exchange-correlation functional provides the best results, while CCSD(T) performs usually worse. Gradient-corrected PBE calculations tend to systematically overestimate metal-benzene binding in the row quintet

Complexes of the neutral ligands methane, propane, and dimethyl ether (DME) with a dimethylcuprate(I) anion (DMCA) were studied using B3LYP and MP2 methods. The quantum theory of atoms in molecule and the second-order perturbation natural bond orbital analysis were applied to analyze the electron density distributions of these complexes and to elucidate the nature of weak closed-shell interactions between the C−H bonds of the ligands and different atoms and bonds of DMCA. The presented results show that the copper center of DMCA interacts with the C−H bonds of methane, propane, and DME via formation of Cu···H−C hydrogen bonds, with the Cu center being an electron charge donor (hydrogen bond acceptor). The formation of weak dihydrogen bonds and C−H···C hydrogen bonds between C−H bonds of the neutral ligands and methyl groups of DMCA additionally stabilizes these complexes. Second-order orbital interactions of C−Cu bonds with C−H bonds contribute also to the formation of the complexes. Each of these interactions is very weak, but the sum of these interactions may have the potential to influence the structures of organocuprates(I), possessing very flat potential energy surfaces.