PROPERTIES OF PD(I)-PD(I) BONDS - THEORETICAL AND SPECTROSCOPIC STUDY OF PD2(DMB)2X2 COMPLEXES (DMB = 1,8-DIISOCYANO-P-MENTHANE, X = CL,BR)

The properties of Pd(I)-Pd(I) bonds and the lowest energy excited states of Pd2(dmb)2X2 complexes (dmb = 1,8-diisocyano-p-menthane; X = Cl, Br) have been investigated from a theoretical and experimental point of view. M2 sigma-bonding interactions are found to arise mainly from interactions between the d(z2) and d(x2-y2) orbitals (as major components) according to extended Huckel molecular orbital calculations (EHMO), contrary to the conventional ''almost pure'' d(z2) M2 interactions found in the prototype M-M sigma-bonded compound Mn2(CO)10. The natures of the MO's are practically insensitive to the change in twist angle (theta; dihedral angle C-Pd-Pd-C) and to the Pd2 bond length. The first- and second-moment analysis of the dsigma --> dsigma+ band indicates that the excited-state distortions (DELTAQ) are very large, even comparable to those of the unbridged Mn2(CO)10. Using the emission spectra (lambda(max) = 625 and 650 nm for X = Cl and Br, respectively) and the oxidation potentials for the first electron (E0/+ = +0.78 and +0.064 V vs SCE for X = Cl and Br, respectively), the excited-state driving forces for electron transfer have been evaluated (1.20 and 1.27 V vs SCE for X = Cl and Br, respectively). The 77 K emission lifetimes (tau(e)) range between 70 and 180 ns for the Pd2(diiso)2X2 complexes (diiso = dmb, tmb; X = Cl, Br). Finally, according to picosecond flash photolysis observations, both photoinduced homolytic Pd2 and PdX bond cleavages appear to be processes occurring in the 3(dsigma --> dsigma*)* states of the investigated compounds. During the course of our studies, r(Pd2) values for the Pd2(dmb)2X2 complexes (X = Cl, Br) have been evaluated to be 2.72 +/- 0.05 angstrom on the basis of vibrational data and an empirical relationship between bond length and M2 force constants for the closely related Pd2(tmb)2Cl2 complex (tmb = 2,5-dimethyl-2,5-diisocyanohexane).