AN INVESTIGATION OF THE M-A-M ANGLE IN COMPOUNDS OF GENERAL FORMULA [ML(N)](2)(MU-A)

The structures of 283 compounds of general formula [ML(m)](mu A)[M'L(n )'] (M, M' = d- or f-block element, L, L' = ligand(s), A = p-block ele ment) have been analysed with respect to the M-A-M angle (theta). Mole cular orbital (primarily extended Huckel) and statistical methods were used for the analysis. It is shown that compounds with 170 < theta < 180 are ''linear''. For all [ML(n)](2)(mu-A) compounds except [(eta-C( 5)R(5))(2)ML(n)](2)(mu-A), the M-A-M interaction consists of cylindric al symmetrical sets of pi- and delta-orbitals derived from the d-orbit als of the M atoms and the p-orbitals on the A atom. Free rotation of the two ML(n) units with respect to one another can occur to minimize steric repulsion across the M-A-M bridge. For [ML(5)](2)(mu-A) compoun ds with 0-8 electrons on the two metal atoms, the theta angle is deter mined by intramolecular steric repulsion of the ML(5) units across the M-A-M bridge. It is predicted that [ML(5)](2)(mu-A) compounds with 9- 12 electrons will be bent (theta < 170 degrees), unless steric repulsi on prevents bending. Compounds of type [(eta-C(5)R(5))ML(2)](2)(mu-A) are a special case of [ML(5)](2)(mu-A) in which rotation of the pyrami dalized (eta-C(5)R(5))ML(2) units occurs to minimize the steric repuls ion across the M-A-M bridge. Similar conclusions apply to [ML(6)](2)(m u-A) compounds with 0-4 or 5-8 electrons. There are three geometries f or [ML(4)](2)(mu-A) compounds: square pyramidal with A in the apical p osition: trigonal bipyramidal with A equatorial; trigonal bipyramidal with A axial. The first type behave like [ML(5)](2)(mu-A). In the seco nd and third cases, steric repulsions across the M-A-M bridge are the main determinant of the theta angle regardless of the electron count. For [ML(3)](2)(mu-A) compounds, the theta angle is determined by inter molecular packing forces unless the ligands are very bulky. The M-A-M interaction in [(eta-C(5)R(5))(2)ML(n)](2)(mu-A) compounds is allylic in nature. Steric repulsion dictates that these compounds have linear M-A-M units. Copyright (C) 1996 Elsevier Science Ltd