Transcript Slide 1
Lecture 20 Dinuclear metal complexes with multiple M-M bonds 1) Transition metal-metal bonding: MO diagram • Compared to main group elements, transition metals (TM’s) have more options for bonding with one another. • In addition to one s- and up to two p-bonds common for organic compounds (acetylenes, cumulenes) and some other main group elements two more bonds of d-symmetry are possible for TM’s (five bonds in total). • • s p g dxz, dyz If the metal dx2-y2 orbitals are involved in s-bonding with ligands, four metal atomic orbitals remain only available to form M-M bonds. Species with TM-TM bond orders 1-4 are well known. Multiple M-M bonds are most typical for such metals as V-Ta, Cr-W, Re, Ru, Os, Rh. g d , d xy x2-y2 g d g d , d xz yz M • d dxy, dx2-y2 M dz2 g p z s g dz2 2) Evidences for M-M double, triple and quadruple bonding • One of the arguments in favor of multiple bonds is M-M bond length (Å): MeO OMe Me O Cl Cl W MeO MeO MeOH Cl Cl W Cl Cl OMe Me O Me MeO Cl MeO MeO O O O O W W O O O O W W OMe HOMe 2.30 2.48 Re Me 2- Me Me Me Me Re Me Me Me Me Me Me • • Cl OMe OMe 2.21 Look now at the solid state structures of salts derived from [Re 2Me8]2- anion which shows the eclipsed conformation of the methyl groups. Me • MeO W W l(W-W)= 2.72 • OMe Me O Me Me Me Me Me not Me R N Mo N Me Me Me R Me N R R R R N N N R R R R R N R Me This unfavorable conformation is possible if stabilized by d xy-dxy d-bonding between two rhenium atoms (l(ReRe)=2.18 Å). The dxy-dxy orbital overlap is impossible in the staggered conformation. The barrier of rotation about M-M quadruple bond was estimated in a dinuclear MoII porphyrin complex to be of 10 kcal/mol. R R Mo N R R 3) More ways to form multiple M-M bonds • The M/M bond order is a function of transition metal electron configuration. The Table below is valid for the case when metal dx2-y2 orbital is used for s-bonding with ligands. O O O H2O d1 d2 d3 d4 d5 d6 d7 d8 Cr O O Cr OH2 O O O l(Cr-Cr)=2.36 1 • • 2 3 3 2 1 0 Some examples are given on the previous page and on the right. A very week quadruple Cr/Cr bond is believed to exist in Cr2(OAc)4(OH2)2 paddlewheel complex above. Another way to form M-M’ multiple bonds is through donor-acceptor mechanism. O PPh2 Ti O O • 4 Ti Rh Rh PPh2 PPh2 l(Ti-Rh) = 2.21 Ti center is roughly tetrahedral (Rh-Ti-O = 109o; sp3 hybridization) while RhP3 fragment is almost planar (d2s hybridization). Rh atom has two filled dxz and dyz orbitals suitable for formation of two p-bonds with Ti atom. The latter uses the empty dxz and dyz orbitals.