Ligands Similar to CO

Data from Y. Mutoh, N. Kozono, M. Araki, N. Tsuchida, K. Takaao, Y. Ishii, Organomerallics, 2010,29,519. 

The complex tra i-[Fe(CO)2(CN)4] can be made by the addition of cyanide to a solution of FeCl2 under an atmosphere of C04  

Dinitrogen is a weaker donor and acceptor than CO. However, N2 complexes are of great interest, especially as possible intermediates in reactions that may simulate natural 

A formal analogy is often drawn between the linear bonding modes of both ligands. NO is isoelectronic with CO therefore, in its bonding to metals, linear NO is considered by electron-counting scheme A as NO, a 2-electron donor. By the neutral ligand method (B), linear NO is counted as a 3-electron donor (it has one more electron than the 2-electron donor CO). 

The bent coordination mode of NO can be considered to arise formally from NO, with the bent geometry suggesting sp hybridization at the nitrogen. By electron-counting scheme A, therefore, bent NO is considered the 2-electron donor NO by the nentral ligand model, it is considered a 1-electron donor. 

Several diatomic ligands similar to CO are worth brief mention. Two of these, CS (thio-carbonyl) and CSe (selenocarbonyl), are of interest in part for purposes of comparison with CO. In most cases, synthesis of CS and CSe complexes is somewhat more difficult than for analogous CO complexes, because CS and CSe do not exist as stable, free molecules and do not, therefore, provide a ready ligand source.Therefore, the comparatively small number of such complexes should not be viewed as an indication of their stability. Thiocarbonyl complexes are also of interest as possible intermediates in certain sulfur transfer reactions in the removal of sulfur from natural fuels. In recent years, the chemistry of complexes containing these ligands has developed more rapidly as avenues for their synthesis have been devised. 

CS and CSe are similar to CO in their bonding modes in that they behave as both o- donors and tt acceptors and can bond to metals in terminal or bridging modes. Of these two ligands, CS has been studied more closely. It usually functions as a stronger or donor and tt acceptor than CO.  

The bent coordination mode of NO is often considered to arise formally from NO-, with the bent geometry suggesting sp2 hybridization at the nitrogen. By 

Neutral Ligand Model (Covalent) Donor Pair Model (Ionic) 

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The trend towards higher linearities breaks down at two instances one involves a rather bulky ligand with a 0-value of 190°, the other one involves hexafluoroisopropyl phosphite that has a very high %-value. Both give rise to unstable rhodium carbonyl complexes either for steric reasons (vide infra) or for electronic reasons. The %-value of hexafluoroisopropyl phosphite is very high indeed and it is thought that this value of 51 means that electronically it is very similar to CO, i.e. a strong electron acceptor. Hence, the propensity of its... 

The PF3 ligand is similar to CO in many aspects of its coordination chemistry, and this is well exemplified by the small area of its osmium chemistry so far explored. The oxidation stages — II, 0 and II are represented, and there is an adduct of osmium(VIII). Little is known about complexes of PC13 with osmium. 

Recently two reports of interaction of dioxygen with Ru " have appeared. The complex of Ru with EDTA appears to form anf-.f dioxygen complex with a bridging OH ligand similar to many Co(III) complexes and Ru " in a zeolite has been shown to react with dioxygen to give Ru and superoxide l... 

Although not an organic ligand, the NO (nitrosyl) ligand deserves discussion here because of its similarities to CO. Like CO, it is both a a donor and tt acceptor and can serve as a terminal or bridging ligand useful information can be obtained about its compounds by analysis of its infrared spectra. Unlike CO, however, terminal NO has two common coordination modes, linear (like CO) and bent. Examples of NO complexes are shown in Figure 13-18. 

Two completely separated P2 ligands originating from P4 activation are present in the previously mentioned complexes [ CpxM 2(/j, 1 1P2)2] (M=Co, Rh, Ir). Orthogonally arranged P2 ligands similar to a diphosphido dumbbell were found in the dodecahedral cluster [ Cp Fe 4(/j, 1 2 2 1-P2)] which is generated via high temperature cothermolysis of [ Cp Fe(CO)2 2] with P4 [80]. 

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