Structure Studies of Dimeric [Pt2(CN)10]4- Pentacyanoplatinum(III) and Monomeric Pentacyanoplatinum(IV) Complexes by EXAFS, Vibrational Spectroscopy, and X-ray Crystallography

Abstract

The structures of the dimeric bispentacyanoplatinate(III)(PtPt) complex [Pt2(CN)10]4- and the pentacyanoplatinum(IV) species [Pt(CN)5(OH)]2-, [Pt(CN)5(H2O)]-, and [Pt(CN)5I]2- have been studied in aqueous solution by the EXAFS technique. A nonsupported Pt−Pt bond, 2.73(1) Å, connects two Pt(CN)5 entities in the dimer. Normal coordinate analyses have been made on the basis of the Raman spectra, and the force constants have been used for discussions of the bonding. The metal−metal stretching force constants for the [Pt2(CN)10]4- complex, the [(NC)5Pt−Tl(CN)n]n- (n = 1, 2, and 3) complexes, and some other dimetallic complexes have been correlated with the metal−metal bond distances. In the [Pt(CN)5X]n- complexes, the mean Pt−C bond distance of the pentacyanoplatinum group was found to increase, and the corresponding Pt−C force constants were found to decrease, with increasing donor ability of the ligand X in the order X = H2O, OH, I, and Pt(CN)5. The crystal structures of the compounds Tl2[Pt(CN)5(OH)] and K2[Pt(CN)5I]·0.6H2O were determined by single-crystal X-ray diffraction techniques and used for comparisons with the EXAFS models. The Pt−I bond distance of the [Pt(CN)5I]2- complex is 2.676(2) Å in the crystal structure and 2.66(1) Å in solution by EXAFS. A method is described for estimating the force constant for the metal−metal bond stretch in dimetallic complexes with heavy metal atoms, based on the use of an effective “spectroscopic” mass of the metal atoms in a diatomic model. The 18-electron rule is found to be useful for rationalizing the structures of the metal−metal bonded cyano complexes and a guideline in searching for metal cyano complexes analogous to those currently described.