Chain conformation and metal center dot center dot center dot metal contacts in dimers and stacks of d(8)-ML4 complexes: Electronic effects

A qualitative theoretical study is presented of the ligand effects on the strength of the M ... M nonbonded interaction in dimers and stacks of d(8)-ML4 chains. It is found that pi-acidic ligands enhance and pi-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 calculations (extended Huckel 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.