Tetranuclear carbonyls of osmium

Article abstract of DOI:10.1016/0022-328X(90)85144-N

Osmium forms more neutral binary carbonyls than any other metal; before 1987, nine were known with one to eight osmium atoms.There were, however, no tetranuclear binary carbonyls of osmium known before this date.This review describes the synthesis of Os4(CO)14 (1), Os4(CO)15 (2) and Os4(CO)16 (3) along with various derivatives of these clusters.Addition of Os(CO)5 to Os3(CO)10(C8H14)2 in hexane affords 2 in good yield.The crystal structure of 2 reveals it to have an unusual planar structure with short (2.775 Angstroem) and long (2.998 Angstroem) peripheral Os-Os bonds (the hinge Os-Os bond length of 2.948 Angstroem is more typical of an Os-Os single bond).The unusual metal-metal bond lengths are rationalized in terms of three-center, two-electron metal-metal bonds so that the short bonds have a bond order of 1.5 and the long bonds an order of 0.5.In this way each osmium atom achieves an 18-electron configuration.Several clusters with essentially the same arrangement of metal atoms have also been synthesized (e.g., Os4(CO)14(PMe3) (4), (η⁵-C5Me5)IrOs3(CO)12).The variable temperature 13C NMR spectra of 2 and 4 indicate that rapid CO-exchange in the equatorial plane occurs in these compounds.Other 62-electron clusters prepared in this study were Os4(μ-H)(CO)14(SnMe3) (8) and Os4(μ-H)2(CO)13(PMe3) (9); whereas 8 has a planar metal skeleton, 9 adopts the more common butterfly arrangement.For these clusters, the planar configuration is adopted when at least one of the metal atoms in the hinge position has four terminal ligands.Refluxing 2 in hexane yields Os4(CO)14 (1), which as expected from polyhedral skeletal electron pair theory has a tetrahedral metal core.Evidence is presented which indicates that in solution the carbonyl ligands in 1 undergo exchange on the infrared time scale.Treatment of 2 in CH2Cl2 at 0 deg C with CO (1 atm) gives Os4(CO)16 (3) in essentially quantitative yield.The crystal structure of 3 reveals it to have a cyclobutane-like Os4 core; the Os-Os bonds are long and range in length from 2.979 to 3.000 Angstroem.In solution at room temperature, 3 readily decomposes to give mainly Os3(CO)12.The much greater stability of the trinuclear cluster suggests that the metal-metal bonding in this cluster should be described in terms of a centrally directed molecular orbital plus edge-bridging molecular orbitals, rather than in terms of two-center, two-electron metal-metal bonds.The structures of Os4(CO)15(L) (L = PF3, PMe3, P(OCH2)3CMe, CNBu^t) have been determined.Only the PF3 derivative has a puckered square arrangement of metal atoms (with long Os-Os bonds) like 3; the other derivatives have a spiked triangular arrangement of metal atoms.The Os-Os bond lengths in the latter clusters range from 2.849 to 2.938 Angstroem.From this study it is concluded that it is the electronic properties of L that dictate which structure a cluster of the type Os4(CO)15(L) adopts.The nonrigid properties of 3 and the Os4(CO)15(L) clusters are also briefly discussed.