Syntheses, structures, and spectra of dinuclear, metal-metal bonded compounds of diplatinum(III) and dirhodium(II)

Abstract

Metal atom dimers containing the (sigma)('2)(pi)('4)(delta)('2)(delta)*('2)(pi)*('4 )configuration (d('7)-d('7)) have a net single bond between them and this bonding is exemplified in the real systems Rh(,2)('4+), Ir(,2)('4+), Pd(,2)('6+) and Pt(,2)('6+). Only for the first and last cases, however, have there been reported examples. In contrast to the many compounds known for Rh(,2)('4+), the results for Pt(,2)('6+) are meager. The syntheses of eight compounds of Pt(,2)('6+) are reported. These compounds are tetra-sulfato and phosphato bridged complexes of dinuclear platinum(III). Of the eight compounds, X-ray crystallographic studies have been made on six. The crystal structures show that five of the six compounds have the shortest known platinum-platinum bonds reported to date. Reactivity studies of these systems with heterocyclic tertiary amines, nucleophiles, and salts to determine the stability of these systems have been carried out. The dirhodium(II) field is relatively more developed. The molecules synthesized today are for specific purposes to aid in the understanding of particular ambiguous fields in the chemistry. One such field is the assignment of the two main transitions in the visible region of the electronic spectrum of dirhodium(II) tetracarboxylates. The assignments for these two transitions has a controversial history. Evidence is presented here which supports recent assignments of the two bands by H. B. Gray and co-workers. This new evidence is from a technique relatively new to metal dimer chemistry, circular dichroism. Circular dichroism gives more information concerning the electronic nature of these systems and consequently aids in band assignments. Two chiral compounds of the dirhodium(II) tetracarboxylato class of complexes have been synthesized and structurally characterized to determine their absolute configuration. These molecules belong in the class II group of chiral compounds which is defined as the symmetric chromophore in a chiral molecular environment. The circular dichroism spectra for the two compounds are interpreted by the use of energy levels from theoretical calculations on these systems and application of the one-electron static coupling mechanism. The results show that three separate interpretations of the spectra all agree with the two assignments of (pi)*(Rh-Rh) (--->) (sigma)*(Rh-O) and (pi)(Rh-O) (--->) (sigma)*(Rh-O).