Carbonyl complexes binary

Most of these complexes obey the 18-electron rule. The cluster compounds Co6(CO)16and Rh6(CO)16 do not obey the rule, however more detailed analysis of the bonding in cluster compounds is necessary to satisfactorily account for the electron counting in these and other cluster compounds. This question is considered in Chapter 13. 

One other binary carbonyl does not obey the rule the 17-electron V(CO)6. This complex, which is pyrophoric and much less thermally stable than the 18-electron Cr(CO)6, is one of a few cases in which strong n acceptor ligands do not succeed in requiring an 18-electron configuration. In V(CO)6 the vanadium is apparently too small to permit a seventh coordination site hence, no metal-metal bonded dimer [which might potentially yield 18-electron (CO)6V-V(CO)6] is ordinarily possible.10 However, V(CO)6 is easily reduced to [V(CO)6] , a well-studied 18-electron complex. 

10In rare gas matrices and experiments involving laser photolysis, evidence has been reported for species that may contain two vanadiums and bridging carbonyls. See Z. Liu, Q. Li, Y, Xie, R. B. King, and H. F. Schaefer III, Inorg. Chem., 2007, 46, 1803, and references therein. 

Exercise 4-4 Verify the 18-electron rule for five of the binary carbonyls [other than 

Binary carbonyl complexes can be synthesized in many ways. Several of the most common methods are described below. 

In contrast to the vast number of mono- and multinuclear binary carbonyl complexes of the transition metals, no isolable binary carbonyls of titanium, zirconium, or hafnium have been reported. 

patents issued to the Barium Steel Corporation in 1957 claim the formation of the heptacarbonyls M(CO)7 (M = Ti, Zr, Hf) as intermediates for the purification of these metals (9,10). In this described refining process, the finely divided metal is treated with CO at 300-400°C and 4-8 atm. The resulting liquid heptacarbonyl compound is then thermally dissociated to the pure metal and CO. The alleged existence of these binary carbonyls seems highly unlikely without supporting evidence. 

A report by Ozin et al. in 1977 describes the formation of Ti(CO)6 via matrix cocondensation techniques (11). This green complex, while not isolated, was characterized by its infrared and ultraviolet-visible spectra. In a pure CO matrix, a color change from green to reddish-brown was observed on warming from 10 K to about 40-50 K. The infrared spectrum of the reddish-brown material showed no evidence for coordinated CO, thus suggesting the extreme thermal instability of Ti(CO)6. 

CARBONYL COMPLEXES NOT CONTAINING xr-BONDED HYDROCARBON LIGANDS 

While the majority of group 4B metal carbonyl complexes contain 7r-bonded hydrocarbon ligands, most notably 17-cyclopentadienyl, recent studies by Wreford and co-workers have led to the identification and isolation of three novel phosphine-stabilized titanium carbonyl complexes (12,13). 

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Ni(CO)4 is the sole binary carbonyl complex of the elements of group 10 that is stable (Table 8.1). However, very few studies in which Ni(CO)4 is used in the preparation of catalysts have been reported [43]. This is probably due to the difficulty of manipulation of Ni(CO)4 and its very high toxicity. However, surface Ni(CO)4 species have been identified after the interaction of CO with highly dispersed supported nickel catalysts prepared by other routes [44]. Recent interest in the use of Ni(CO)4 has focused on the controlled production of nickel nanoparticles for specific purposes, such as in automotive converters [45]. The use of nickel tetracarbonyl as an agent for the nucleation process in the growth of single-wall carbon nanotubes has also been reported [46]. 

Other ligands compatible with aqueous conditions and providing additional M-C bonds are CO and isonitriles. As discussed earlier, at ambient pressure of CO, no more than three COs will coordinate to Tc(I) as a consequence of the trans effect. At elevated pressure, however, it is possible to synthesize the binary carbonyl complex [99mTc(CO)6]+ [42]. This could be shown by "Tc NMR spectroscopy and by isolation of the complex. 

An interesting feature of the structures of binary carbonyl complexes is that the tendency of CO to bridge transition metals decreases in going down the periodic table. For example, in Fe2(C0)9 there are three bridging carbonyls, but in Ru2(CO)9 and Os2(CO)9 there is a single bridging CO. A possible explanation is that the orbitals of bridging CO are less able to interact effectively with transition metal atoms as the size of the metals increases. 

MAIN GROUP PARALLELS WITH BINARY CARBONYL COMPLEXES... 

Main Group Parallels with Binary Carbonyl Complexes 557... 

Binary carbonyls, containing only metal atoms and CO, are fairly numerous. Structures of some representative binary carbonyl complexes are shown in Figure 4-5. 

Chemical similarities occur between main group and transition metal species that are electronically equivalent (i.e., species that require the same number of electrons to achieve a filled valence configuration).28 For example, a halogen atom, one electron short of a valence shell octet, may be considered electronically equivalent to Mn(CO)5, a 17-electron species one electron short of an 18-electron configuration. In this section we discuss briefly some parallels between main group atoms and ions and electronically equivalent binary carbonyl complexes. 

Theoretical organometallic crystal structures have been calculated for the simple binary carbonyl complexes Ni(CO)4, Fe(CO)5, and Cr(CO)6 as representatives of the elasses of tetrahedral, trigonal bipyramidal, and octahedral complexes. It has always been possible to retrace, among other solutions of similar cohesive energy, the experimentally determined crystal structures. For the dinuclear complexes Co2(CO)g and Fe2(CO)9, " the similarity between the moleeular arrangements in the crystals of the two species arises from the stereoactivity of the lone pairs on the Co species which occupy the space taken by the two additional carbonyl ligands in the structure of the Fe eomplex. 

F.5 Chemistry of heterometallic clusters prepared by condensation of Group 6 metal acetylides and Group 8 binary carbonyl complexes... 

Palladium(O) compounds have a configuration and unlike most transition metals this oxidation state is dominated by phosphine complexes rather than carbonyls. In fact binary carbonyl complexes with palladium are unstable at room temperature. The highest coordination number known for Pd is four and [PdLj complexes adopt a square planar structure. Dissociation of ligands from [PdLJ occurs readily to generate the 16- and 14-electron species [PdLs] and [Pdl ] these are trigonal planar and linear respectively. Another notable feature of Pd° is that facile oxidation to cr Pd occurs. 

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