Carbonyls, metal semibridging

Carbonylation metal salt reduction by CO at high pressure with M-CO bond formation Polyhydride complex of the type L MHj (x > 3) Semibridging CO unsymmetrically bridged CO... 

Weakly semibridging carbonyls, such as those in Os4(CO),4, are nearly linear but show some distortion toward the second metal. Of course it is not always clear whether departures from linearity arise from interaction with the metal or because of packing forces in the crystalline solid. All CO ligands are fully nonbridging in the puckered 034(00),, described as a metal carbonyl analogue of cyclobutane. 5 The dark red crystals of Os4(CO) j possess nearly planar OS4 units with only terminal CO ligands. 

The metallic isosceles triangle (Pdl-Pd2, Pd2-Col, 2.55 A Pdl-Col, 2.62 A) is capped at opposite sides by carbonyl ligands. At each side, one of the Pd-C(capping) distances results in semibridging carbonyls (Pdl-C51,2.36 A Pd2-... 

The structure of [FeCo(CO)g] , with idealized Cg symmetry, is closely related to that of the lowest energy M2(CO)g isomer (which has Z)2d symmetry) but does not correspond to a minimum of the PES computed from steric forces alone (Lauher, to force the calculation toward the experimental stereogeometry, had to constrain the Fe-Co-C angle of the semibridging carbonyl group to its experimental value). As a matter of fact, the sterically favored D2d structure, when the metals contribute a different number of valence electrons, is highly polarized and distortion must occur to quench charge polarization partially (Fig. 2). 

Cotton recognized the existence of two broad classes of unsymmetrical bridging carbonyl groups Compensating sets which are distributed in a cyclic fashion over two or more equivalent metal atoms and semibridging carbonyl groups which connect nonequivalent metal atoms. 

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