FORMATION OF CIS-BIS(CARBONYL)PALLADIUM(II) FLUOROSULFATE, CIS-PD(CO)2(SO3F)2, AND ITS CRYSTAL AND MOLECULAR-STRUCTURE

An isomeric mixture of bis(carbonyl)palladium(II) fluorosulfate, Pd(CO)2(SO3F)2, is recrystallized from fluorosulfuric acid, HSO3F, and yields after short, intermittent heating of the solution to 60-degrees-C a single crystal of cis-bis-(carbonyl)palladium(II) fluorosulfate, cis-Pd(CO)2(SO3F)2, which crystallizes in a monoclinic unit cell, space group P2(1)/n, with Z = 4, a = 7.3697(11) angstrom, b = 14.7742(35) angstrom, c = 8.3237(21) angstrom, beta = 90.52(2)-degrees, V = 906.3 angstrom3, and T = 220 K. The refinement converges at R(f) = 0.027 and R(wf) = 0.034. The molecular structure reveals square planar coordination of palladium with terminal CO groups and monodentate fluorosulfates in a cis-geometry. Consistent with the unusually high CO-stretching frequencies (nu(av)BAR = 2218 cm-1), the CO bond lengths are short-1.106(6) and 1.114(6)angstrom. This suggests substantially reduced pi-back-donation, with CO functioning predominantly as a sigma-donor. In addition there are weak C...O contacts in the range 2.819(6)-3.172(6) angstrom, which involve the two noncoordinated oxygens of the fluorosulfate groups. In the absence of significant Pd to CO pi-back-donation, secondary contacts involving the electrophilic carbon atom of the CO group as acceptor appear to provide for some degree of charge compensation and to exert a stabilizing influence on the structure. The importance of such secondary contacts to the bonding in nonclassical carbonyls appears not to have been recognized previously.