FULL PAPER z Chain Conformation and Metal Metal Contacts in Dirners and Stacks of d8-ML, Complexes: Electronic Effects** Gabriel Aull6n and Santiago Alvarez" Abstract: zyxwvutsrqpon A qualitative theoretical study is presented of the ligand effects on the strength of thc M zyxwvutsrqpo . . . M nonbonded interaction in dimers and stacks of d8-ML, chains. It is found that macidic ligands enhance and n-basic ligands weaken the M . . . M bonding interaction. A large amount of structural data can be rationalized with the help of a simple orbital model. Other features of the crystal structures of such compounds are correctly accounted for by semiempirical molecular orbital and electronic band calcula- tions (extended Hiickel level). Electronic effects control the deviation from linearity of stacks of complexes with mixed ligands, whereas coupled electronic and steric effects determine the conformational preference. The predictive capability of theoretical band calculations is found to be good. Introduction The enormous intercst devoted to stacked square-planar d8- ML, complexes and to their partially oxidized derivatives dur- ing the last decades is undoubtedly associated with the possibil- ity of developing one-dimensional systems showing electrical conductivity." - 61 In fact, the electrical conductivity along the chain direction in these compounds is several orders of magni- tude larger than in the perpendicular directions." '. The formation of clcctronic bands associated with the d,, metal orbitals was studied by Whangbo and H~ffmann['~ for the case of the tetracyanoplatinate(I1) salts, focusing mainly on the bonding and metallic conductivity of the partially oxidized derivatives. Similar studies were reported for the chains of dithi- olene and dioximato square-planar d8 How- ever, the factors that control other structural aspects of these systems, and the influence they might have on optical or electri- cal properties, have not been systematically studied from a the- oretical point of view. Among these factors are the nature of the ligands, the relative rotation of two successive monomcrs in a chain (measured by the torsion angle zyxwvuts z shown in Scheme I), and the degree of linearity of the chain (defined by the M-M-M angle 6 in Scheme 1). In this paper we report a theoretical study of the structural aspects mentioned above based on molecular orbital calcula- [*I S. Alvarez, G. Aullhn Departainent de Quimica Inorganica, Universitat de Barcelona Diagonal 647, 08028 Barcelona (Spain) Fax: Int. code +(3)490-7725 e-mail: salvarez(cbkripto.qui.ub.es [**I The abbreviations used in this paper are given in the Appendix Keywords band theory * electronic structure * stacking interactions - square-planar coniplexes - one-dimensional com- plexes Scheiiie 1. Parameters defining the geometry of a cham of square-planar d"-ML, complexes tions for dimers and band structure calculations for infinite chains, carried out at the extended Huckel level. The only theo- retical background that is needed to express the results of our work is a simple qualitative orbital mode1.[12.i31 This model summarizes the metal-metal interaction in the dimers of d8- ML, complexes as the combination of a four-electron rcpulsion between the d,, orbitals of the two metal atoms (labeled (Q in Scheme 2) and the two attractive donor-acceptor interactions Scheme zyxwvutsr 2. Model for metal-metal interactions in a dimer of d"-ML, complexes. Clien?. Eur. J 1997. zyxwvutsrqponmlk 3. No. 4 zyxwvutsrqpon (0 VCH zyxwvutsrqpo ~~rlu~~s~e~~ll,~clif~ft mhH, zyxwvutsrqp 0.69451 U/crnhrlm, 1997 0947-~.r39197!0304-06.(5 5 17 50+ .50/0 655