Metal carbonyls bridged

The CO molecule can be bonded in a terminal or bridged fashion. The metal-carbonyl bridges are not always symmetric and. in some cases, the CO can be considered a four-electron donor, as in Mn2(CO)s(PhjPCH>PPli2) ... 

Structure. The CO molecule coordinates in the ways shown diagrammaticaHy in Figure 1. Terminal carbonyls are the most common. Bridging carbonyls are common in most polynuclear metal carbonyls. As depicted, metal—metal bonds also play an important role in polynuclear metal carbonyls. The metal atoms in carbonyl complexes show a strong tendency to use ak their valence orbitals in forming bonds. These include the n + 1)5 and the n + l)p orbitals. As a result, use of the 18-electron rule is successflil in predicting the stmcture of most metal carbonyls. 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

M ,(CO) Metal carbonyl clusters with bridging CO groups M-C(0) M... 

Mathur, Pradeep, Chalcogen-Bridged Metal-Carbonyl Complexes. 41 243... 

A monocarbonyl complex of zirconocene has been proposed by Floriani and co-workers as a transient intermediate in the carbonylation of the bridging oxymethylene complex (Cp2ZrCl)2(Ai-CH20) (57) (104,105). The assignment of 58 as the proposed intermediate to the final carbonylation product, [Cp2Zr(/i-CHO=CHO)]2 (59), was based on the appearance of a metal carbonyl band at 1970 cm-1 together with the subsequent CO-induced loss of Cp2ZrCl2. 

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. 

The structures of the dinuclear metal carbonyls (containing two metal atoms) involve either metal-metal bonds or bridging CO groups, or both. For example, the structure of Fe2(CO)9, diiron nonacar-bonyl, contains three CO ligands that form bridges between the iron atoms, and each iron atom also has three other CO groups attached only to that atom. 

However, these reactions jure associated with terminal rather than bridging carbonyl groups. In metal "ketonic compounds, the center for attack is normally the metal. This may be rationalized in terms of additional delocalization of the positive charge from the carbon to the metal center and implies a "carbyne contribution to the carbonyl bridge, of the form discussed above for the methylated products, again emphasizing the possibility that in systems of this type reaction may occur at either the metal or ligand center... 

The experimental evidence shows that the carbon of terminal carbonyl groups is positively polarized (or polarizable) and, contrary to the behaviour of free carbon monoxide, is easily attacked by strong nucleophiles (OH-, OR-) a behaviour which is general in the chemistry of metal carbonyls. Moreover, the negative polarisation (or polarizability) of oxygen atoms of carbonyl groups, particularly bridging carbonyls, is illustrated by the facile formation of adducts with Lewis acids as shown inEq. (16) 7. ... 

There are two principal modes of coordination of the carbon monoxide ligand in transition metal carbonyls terminal coordination, to a single metal atom, and bridging coordination, to two or more metal atoms. The Ols spectrum of Co4(CO)12, shown in Fig. 5, can be readily deconvoluted into two peaks corresponding to these two types of carbonyl groups109. This spectrum is useful for determining the relative chemical shifts for the two types because... 

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