CHEMICAL APPLICATIONS OF TOPOLOGY AND GROUP-THEORY - 30 - DISTORTIONSFROM IDEALIZED SYMMETRICAL COORDINATION POLYHEDRA BY LIFTING MOLECULAR-ORBITAL DEGENERACIES

Transition metal complexes with orbitally degenerate electronic states are geometrically unstable with respect to distortion from idealized symmetry to give structures with non-degenerate electronic states in a ccord with the Jahn-Teller theorem. This paper examines such distortio ns for metal complexes of the general type MLn (4 less than or equal t o n less than or equal to 9) having open-shell electronic configuratio ns, Such distortions can be of the following two types: (i) a classica l Jahn-Teller distortion in which the topology of the polyhedron is ma intained but the symmetry is lowered from a point group G to a smaller point group H where G is a subgroup of H; (ii) distortions in which b oth the topology and symmetry of the polyhedron are changed, typically by diamond-square processes, from a point group G(1) to G(2) where G( 2) is not necessarily a subgroup of G(1). Certain octahedral metal com plexes ML6 deviating from the favored closed-shell 18-electron rare ga s electronic configuration are observed to be geometrically unstable t o both types of distortions, e.g., elongation of a hyperelectronic O-h octahedron to a D-4h square bipyramid in octahedral d(9) Cu(II) compl exes as compared with trigonal twists of hypoelectronic early transiti on metal dithiolates (e.g., Mo[S2C2(CF3)(2)](3)) and hexamethyls (e.g. , W(CH3)(6) and Zr(CH3)(6)(2-)) to give D-3h or C-3v trigonal prismati c complexes. Open-shell tetrahedral metal complexes may similarly be u nstable to both types of distortions, e,g, elongation or compression o f opposite edges of a T-d tetrahedral complex ML4 to give a D-2d disph enoidal complex as compared with a 90 degrees twist of a d(8) tetrahed ral complex to give a D-4h square planar complex, Similar geometric in stabilities of open-shell penta-coordinate metal complexes may be reso lved by partial diamond-square distortions to intermediate points alon g the trigonal bipyramid/square pyramid continuum to remove an of the orbital degeneracies. Analogous diamond-square processes in open-shell superoctahedral complexes MLn (n=7, 8, 9) along the pentagonal bipyra mid/capped octahedron, bisdisphenoid/square antiprism, and tricapped t rigonal prism/capped antiprism continua, respectively, are not suffici ent to remove all degeneracies. However, neither hypoelectronic nor hy perelectronic superoctahedral metal complexes are expected to be stabl e. (C) 1998 Elsevier Science S.A.