# Structure and Bonding in Cyclic Isomers of $BAl_2H_n ^ m$ (n=3–6, m= -2 to +1):Preference for Planar Tetracoordination, Pyramidal Tricoordination, and Divalency

Jemmis, Eluvathingal D and Parameswaran, Pattiyil (2007) Structure and Bonding in Cyclic Isomers of $BAl_2H_n ^ m$ (n=3–6, m= -2 to +1):Preference for Planar Tetracoordination, Pyramidal Tricoordination, and Divalency. In: Chemistry - A European Journal, 13 (9). pp. 2622-2631.

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## Abstract

The structure and energetics of cyclic $BAl_2H_n ^ m$ (n=3–6, m= -2 to +1), calculated at the $B3LYP/6-311+G^{**}$ and $QCISD(T)/6-311++G^{**}$ levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of $BAl_2H_3^{-2}$ and $BSi_2H_3$. The number of structures that contain hydrogens out of the $BAl_2$ ring plane is found to increase from $BAl_2H_3^{-2}$ to $BAl_2H_6^{+}$. Double bridging at one bond is common in $BAl_2H_5$ and $BAl_2H_6^{+}$ . Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of $BAl_2H_3^{-2}$. $BAl_2H_4^{-}$ (2b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the $B-Al$ bond prefer not to be in the $BAl_2$ plane so that the \pi MO is stabilised by $\pi-\sigma$ mixing. This stabilization increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between $B-Al$ bonds. The relative stabilisation energy (RSE) for the minimum energy structures of $BAl_2H_n^m$ that contain \pi -delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.

Item Type: Journal Article Copyright of this article belongs to John Wiley and Sons, Inc. ab initio calculations;aluminum;boron;bridging hydrogen;protonation. Division of Chemical Sciences > Inorganic & Physical Chemistry 28 Jul 2008 19 Sep 2010 04:48 http://eprints.iisc.ernet.in/id/eprint/15270