Carbonyl ligands terminal

A rare example of thiourea coordination to low-valent Co is of a disubstituted thiourea as bridging ligand, observed in the cluster Co3(CO)7(/i3-S)(/i- 72-PhNC(S)NHCH2Ph) which is formed by reaction of Co2(CO)8 with the thiourea.172 The crystal structure of the product defines a tetrahedral Co3S core with all carbonyls in terminal positions and the deprotonated thiourea bridging two Co centers via the S and an amido N. 

In 1982, a most remarkable reaction was discovered by Bercaw et al. which involved the reductive coupling of the terminal carbonyl ligands of [CpFe(CO)2]2- When 1 equivalent of the permethylated zirconocene dinitrogen complex [(rj-CsMe Zr k (29) was treated with 2 equivalents of [CpFe(CO)2]2 in toluene above -20°C, 3 equivalents of N2 evolved... 

While essentially all the metal carbonyl complexes for group 4B contain terminal CO ligands, only recently have some bonafide doubly bridging carbonyl complexes been reported. However, these complexes are hetero-nuclear, since the carbonyl ligand bridges a zirconium atom with the metal center of a late transition metal. 

Another route to the formation of a hydrazide on a surface is to use an aldehyde-containing particle (such as HEMA/acrolein copolymers) and subsequently modify the aldehydes to form hydrazone linkages with bis-hydrazide compounds, which then can be stabilized by reduction with sodium cyanoborohydride (Chapter 2, Section 5). The resulting derivative contains terminal hydrazides for immobilization of carbonyl ligands (see Figure 14.18). 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

Use of Co2(CO)8 in reactions involving 1,2-propadienes remains for the most part unexplored. It has been reported that terminal 1,2-propadienes react with Co2(CO)8 to form unidentified complexes, and that excess 1,2-propadiene is polymerized concurrently [30]. It has also been reported by Nakamura that a novel dimeric complex 54, in which a carbonyl ligand is connected to the central carbon of 1,2-propadiene, is produced by the reaction of 1,2-propadiene itself with Co2(CO)8 (Scheme 23) [31]. However, unlike the well-known chemistry of alkyne-Co2(CO)6 complexes, these 1,2-propadiene-cobalt carbonyl complexes have rarely been applied in synthetic reactions, probably due to their high activity in catalyzing the polymerization of 1,2-propadienes [32]. 

Dodecacarbonyltriruthenium is an orange, air- and light-stable crystalline solid. It is soluble in most organic solvents. Its infrared (IR) spectrum in hexane displays three bands attributable to terminal CO ligands 2061 (vs), 2031 (s), and 2011 (m)cm 1. No band assignable to a bridging carbonyl ligand is observed. 

Carbonylchlorocopper(I) is a colorless crystalline substance that decomposes rapidly in the absence of a carbon monoxide atmosphere to give copper(I) chloride and carbon monoxide. The compound is, however, stable for long periods of time if stored under carbon monoxide. Cu(CO)Cl has a polymeric structure,10 which may be described as layers of fused, six-membered, copper-chloride rings in the chair conformation, with terminally bonded carbonyl ligands. The infrared spectrum of Cu(CO)Cl (Nujol mull at 0°C) displays a characteristic large peak at 2127 cm -1 and a vibrational analysis has been reported.13... 

This reduction in symmetry of the metal array occurs in Pdi0( i3-CO)4(p-CO)g(PBu3)6,304 where there is also a lower symmetry set of ligand connections. There is still a (Pd )6 octahedron, capped by (p3-CO) on four tetrahedrally arrayed faces, but the Pd° are located unsymmetrically over the remaining faces, being connected by doubly-bridging carbonyls to the equatorial Pd only. The Pd° thereby constitute a disphenoid rather than a tetrahedron. Phosphine ligands terminate the Pd° and the two axial Pd . 

The thermolysis of 4 with Ru3(CO)12 3 in bis(2-methoxyethyl)ether under reflux for 3 h affords the decanuclear cluster [RuioC(CO)24]2 218 in 81% yield (Scheme 36).124 The same cluster has also been isolated from the thermolysis of 3 with mesitylene.95 The structure of 218 has been established by single crystal X-ray diffraction, which shows that the metal skeleton consists of a tetra-capped octahedron decorated with terminal carbonyl ligands. Cluster 218 reacts with CO in dichloromethane under ambient conditions to regenerate 4 and 3 in quantitative yield.109 The decanuclear cluster 218 also undergoes a reversible reaction with two equivalents of iodine to afford [RuioC(CO)24I] 219.109 At higher temperatures further reaction occurs with iodine to produce a species tentatively characterized as the hexamer [Ru6C(CO)i6I2] 220. 

The electron impact-induced decomposition pathways of several structurally related /u.-methylene complexes of cobalt, rhodium, manganese, and iron have been elucidated by high resolution measurements, analysis of metastable transition (DADI linked scan), and 2H labeling (46). Terminal carbonyl ligands are generally lost prior to further fragmentation of the three-membered frameworks. Subsequent rearrangement reactions of the... 

While the lowering of the carbonyl stretching frequency reflects a reduction in the C—O force constant and bond order, the resultant carbonyl ligand can hardly be construed as activated. In general the terminal M—CO unit is unreactive to most reagents. One of the reasons for this lies in the fact that the bond order in a terminally bonded CO is not greatly reduced, i.e., the CO triple bond has not been reduced beyond a bond order of 2, with the exact value depending on the particular compound. 

A different reason for the lack of reactivity of terminal carbonyl ligands is based on the notion that an unreactive charge distribution results from the interaction that leads to the bond order reduction. This view requires... 

Additional activation of coordinated CO is possible in strongly acidic media in which protonation of the terminal oxygen lone pair can further enhance the charge polarization of the carbonyl ligand and the susceptibility of the C atom to undergo nucleophilic attack. While stable species showing this type of binding have not been isolated, their possible kinetic role cannot be ruled out. 

---