~ ) Pergamon PIh S0277-5387(96)00573-6 Polyhedron Vol. 16, No. 14, pp. 2435-2440, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277-5387/97 $17.00+0.00 Complexes of main-group elements containing stereochemically active lone pairs: the crystal structures of [TI(TpA")] {Tp An = tris[3-(2- methoxyphenyl)pyrazol-l-yllhydroborate} and [Pb(TpPY)(NO3)] • 0.5Et20 { T p Py ---- trisl3-(2- pyridyl) pyr azol- 1-yl] hydrobor ate } Peter L. Jones, Karen L. V. Mann, John C. Jeffery, Jon A. McCleverty* and Michael D. Ward* School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 ITS, U.K. (Received 23 October 1996; accepted 15 November 1996) Abstract--The new podand ligand tris[3-(2-methoxyphenyl)pyrazol-l-yl]hydroborate (Tp A°) has been pre- pared and the crystal structure of [TI(TpAn)] determined. The metal ion is in a trigonal pyramidal geometry arising from the three pyrazolyl N-donors, with the lone pair of the T11 ion in the fourth position of the tetrahedron of electron pairs. There is also a weak but significant interaction with one of the anisyl oxygen atoms (T1...O = 3.018 ~), and the displacement of the metal ion towards this oxygen atom results in a noticeable lengthening of the TI--N(pyrazolyl) bond which lies approximately trans to it (2.70 &, compared with 2.54/~ for the other two T1--N bonds). The crystal structure of [Pb(TpPY)(NO3)] "0.5Et20 shows that the Pb n ion has four strong interactions (with the pyrazolyl nitrogen atoms and one of the nitrate oxygen atoms; range of bond lengths 2.49-2.67 A) and four weak ones (with the pyridyl nitrogen atoms and the other nitrate oxygen atom; range of bond lengths 2.80-2.99 ,~). If only the four stronger interactions are considered, the irregular coordination sphere of the Pb H ion has an obvious gap in it which is the site of the stereochemically active lone pair. © 1997 Elsevier Science Ltd Keywords: tris(pyrazolyl)borate; lead; thallium; crystal structures. As part of our studies on the coordination behaviour of podand ligands derived from tris (pyrazolyl)borates we have become interested in the structures of com- plexes with non-transition-metal ions [1-4]. One can see how the coordination behaviour of the ligands is determined by the size and charge alone of the metal ions, without competing stereoelectronic preferences arising from the partially filled d-shell of the metal ions. Our recent studies have concentrated on the hexadentate N-donor podand tris[3-(2-pyridyl) pyrazol-l-yl]hydroborate (TpPY). In [TI(TpPY)] for example the T1 ~ ion is coordinated principally by the three pyrazolyl donors, with much weaker inter- actions with the remote pyridyl donors; in contrast, *Authors to whom correspondence should be addressed. using Ag~--which is of a similar size and has the same charge as Tl~--the unusual cluster complex [Ag3(TpeY)2 ] + forms in which a triangular cluster is encapsulated by two podand ligands [1]. Lanthanide [2,3] and actinide [4] ions are large enough to occupy the cavity fully resulting in donation of all six N- donor atoms to a single metal centre. In this paper we describe the crystal structure of the Pb" complex of this podand, [Pb(TpPY)(NO3)]. In an effort to prepare related podand ligands with different donor sets, we have also synthesized tris[3- (2-methoxyphenyl)pyrazol-l-yl]hydroborate (Tp A", where the suffix "An" denotes the 2-anisyl sub- stituents on the pyrazolyl rings). This could act as an N303 donor if the ether oxygen atoms coordinate or just as a conventional terdentate tris(pyrazolyl)borate with bulky aryl substituents shielding the metal centre 2435