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Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
Before we start with 18-electron rule, let us first recall the Octet rule. The Octet rule states that atoms such as carbon with one 2s and three 2p orbitals require eight electrons to completely fill the valence shell and compounds, which can fulfill this requirement are left with no or little tendency to participate in chemical reactions, thus achieve stability. There is a similar tendency with metals to react with ligands forming organometallic or coordination complexes in such a way as to attain the electronic configuration of next inert gas in the series [ns2(n−1)d10np6]. The 18-electron rule states that a metal complex with 18 electrons (filled valance shell or completes the insert gas configuration) in its outer shell will be stable as compared to the metal complexes either less than or more than 18 electrons. This is just a rule of thumb and there can be many exceptions to this rule; nevertheless, it offers guidelines to the chemistry of coordination and organometallic compounds. For example, if there is a competition of any given metal with two sets of ligands, one which yields the complex 18-electron configuration and the other less or more than 18-electron configuration, then the metal will prefer the former ligand over later due to the obvious reason of stability. The valence electron count of a complex includes the valence electrons of the central metal ion and the electrons shared or donated by the ligands, and for inert gas configuration it comes to 18. Counting the valence electrons in a complex allows us to predict the stability associated with complex and also to predict the mechanism and mode of reactivity to achieve 18-electron configuration.
Metal sandwich and ion complexes in cyclacene nanobelts
Published in Molecular Physics, 2023
Sandwich compounds are a class of molecule in which a central metal atom or ion forms haptic covalent bonds to two arene ligands located on either side of the metal centre. Bis(benzene)metal complexes are sandwich compounds with the molecular formula M(η6-C6H6)2, where M represents the central metal that bonds with two different benzenoid rings. Bis(benzene)chromium, together with its isoelectronic compounds, are known to form stable structures. This can be rationalised as the η6 electron donation from the two benzenoid ligands provides the 12 electrons needed to meet the 18-electron rule of thumb in transition metal chemistry. This ‘rule’ arises as 18 electrons are needed to fill up the valence s, p, and d-orbitals of a transition metal, creating a particularly stable overall electronic structure. The stability of these free standing metal-benzenoid complexes raises the question of whether transition metal sandwich complexes can form internally within cyclacene nanobelts by bridging the two internal benzenoid faces of the central cavity, and cyclacene belts isomers with both neutral and ionic transition metal centres have been studied here to explore this possibility.
Theory of chemical bonds in metalloenzymes XXIV electronic and spin structures of FeMoco and Fe-S clusters by classical and quantum computing
Published in Molecular Physics, 2020
Koichi Miyagawa, Mitsuo Shoji, Hiroshi Isobe, Shusuke Yamanaka, Takashi Kawakami, Mitsutaka Okumura, Kizashi Yamaguchi
In 1980s, transition-metal complexes with C3 and Td structures were found in native iron-sulfur complexes [69,70]. Moreover, the trinuclear sulfur-centred iron complexes [Fe3S4(SR)3] were synthesised as artificial models for biological iron-sulfur enzymes as illustrated in Figures 1 and 5. The triangular Fe3 skeleton was consisted of the tetrahedral ligand coordination, but eighteen (18) electron rule for closed-shell transition metal complexes was broken, providing unpaired 3d electron spin(s) at the Fe site. Therefore, the Heisenberg spin Hamiltonian in Equation (4a) was applicable for exchange-coupled local spins on the Fe ion. The tetrahedral or cubane-type skeleton was also realised in the case of Fe4S4 complex [26,69,70] because of the ligand coordinations in sharp contrast to the organic radical clusters [64–66,72]. Thus, it has been found that axial (one-dimensional;1D), helical (2D) and torsional (3D) spin structures are feasible for Fe-S clusters; 1D structure for 2, 3 and 4, 2D structure for 5 and 6, and 3D structure for 7, 8 and 9, as illustrated in Figures 1 and 5. The singlet-type spin-coupling fraction become the maximum for these spin structures. Thus, the spin vector models are very useful for theoretical understanding and prediction for possible spin structures of Fe-S clusters such as 14 and 16 in Figure 1.
Superatomic properties of transition-metal-doped tetrahexahedral lithium clusters: TM@Li14
Published in Molecular Physics, 2020
Lijuan Yan, Jun Liu, Jianmei Shao
These features indicate that a transition-metal atom embedded into Li14 cage tends to form superatoms, of which the stability is effected by the electronic and geometric shells [44,45]. On the basis of the jellium model, the stability of TM@Li14 (TM = Ti and W) is enhanced by their closed electronic and geometric shells, filling the 18-electron rule and 20-electron rule, respectively. For TM@Li14 (TM = Sc and V), both with a partially filled subshell, they can stabilise themselves via spin multiplicities. Correspondingly, the total effective valence electrons are between 17 and 20, and the total magnetic moment is changing between 0 with 1 μB for TM@Li14 (TM = Sc, Ti, V and W) clusters, exhibiting a remarkable odd-even alternation, as shown in Figure 3. The findings are developing with binary lithium superatoms compared with the previous researches of VLin (n = 1–13) clusters reported by Zhang et al. [29] and of M@Li16 (M = Ca, Sr, Ba, Ti, Zr, Hf) reported by Gu et al. [14]. However, not all the transition-metal doped TM@Li14 clusters are suitable for the D6d symmetry superatomic structures due to the number of valence electrons and the mismatch of atomic radius. That’s why the lowest energy structures of TM@Li14 clusters using Cr, Mo and W atoms as dopants, which have the same effective valence electrons, are diverse as shown in Table 1.