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A convenient approach to obtain Lewis structures for compounds of the type YXn involves first constructing a trial structure that satisfies the valence of the outer atoms (e.g. 1 bond for fluorine, 2 bonds for oxygen and 3 bonds for nitrogen) and placing the molecular charge (if any) on the central atom. The second step involves evaluating the electron count of the central atom, which can give rise to three possibilities: (i) if the central atom has an octet configuration, no change in the number of bonds is required, (ii) if the central atom (Y) exceeds the octet, a Y–X bond is relocated as a lone pair on X, which results in a formal positive charge on Y and a formal negative charge on X, and (iii) if the electron count on the central Y atom is less than an octet, a lone pair on the outer atom is relocated as a Y–X bond, which results in a formal negative charge on Y and a formal positive charge on X; these transformations modify the electron configuration around X such that it will adopt a correct Lewis structure. This approach differs considerably from other methods that require one to first calculate the total number of valence electrons. As such, the method described here, which focuses on using valence as a guiding chemical principle, is much less mathematically oriented and therefore less subject to errors from incorrect calculations. 

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl thallium compound, [TismPriBenz]Tl, reacts with Li, Na, K and Rb afford the respective alkali metal carbatrane complex, [TismPriBenz]M (M = Li, Na, K, Rb), while the cesium complex [TismPriBenz]Cs is obtained via the reaction of [TismPriBenz]Li with CsF. These carbatrane compounds [TismPriBenz]M (M = Li, Na, K, Rb and Cs) constitute the first complete series of structurally characterized monomeric alkyl compounds of the nonradioactive alkali metals. 

The covalent bond classification (CBC) method represents a molecule as ML l X x Z z by evaluating the total number of L, X and Z functions interacting with M. The CBC method is a simplistic approach that is based on the notion that the bonding of a ligating atom (or group of atoms) can be expressed in terms of the number of electrons it contributes to a 2-electron bond. In many cases, the bonding in a molecule of interest can be described in terms of a 2-center 2-electron bonding model and the ML l X x Z z classification can be derived straightforwardly by considering each ligand independently. However, the bonding within a molecule cannot always be described satisfactorily by using a 2-center 2-electron model and, in such situations, the ML l X x Z z classification requires a more detailed consideration than one in which each ligand is treated in an independent manner. The purpose of this article is to provide examples of how the ML l X x Z z classification is obtained in the presence of multicenter bonding interactions. Specific emphasis is given to the treatment of multiple π-acceptor ligands and the impact on the v n configuration, i.e. the number of formally nonbonding electrons on an element of interest.