Illustrations of the Isolobal Analogy

However, in the isoelectronic Ru3(CO)i2 and Os3(CO)i2, all carbonyls are terminal [8]. Again, this implies a very small energy difference. The structure shown for Cp3Rh3(CO)3 is one of several possibilities [9] that pass through an all-terminal CO structure. Mixed structures such as 21.37 [10] and 21.38 [I I] also have been 

The simplest, stable organic compound containing the CH2 group is ethylene. It is reasonable to expect that the compounds given by 21.42-21.45 can be made. 

Derivatives of 21.42 have been prepared [ 15], and details concerning 21.43 in a low temperature matrix have been described in Section 19.3.21.45 has been structurally categorized as the pentamethylcyclopendienyl derivative (see 16.29). Their kinetic stability is certainly limited. A high lying 7t orbital and low-lying n especially in 21.42- 

45 fragments are coordinated to a single Pt atom in 21.46 [16]. This molecule is isolobal to (ethylene)2Pt The Os2(CO)8 analog of 21.43 appears in 21.47 [17]. Replacing the Os(CO)4 groups with isolobal (indenyl)Rh(CO) and CH2 units, and the 

So far we have concentrated on isolobal analogs of CH2. Let us now turn our attention to other fragments. In Section 17.4, we showed thatforthe hydrido bridged Cr(CO)s dimer, 21.50, a closed structure with a Cr— H—Cr angle of less than 180° 

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This property of gold(I) moieties to form partially bonding contacts to other gold species has been described as aurophilicity by Schmid-baur and his co-workers (266) and represents an important limitation of the isolobal analogy. An example of this is provided by a comparison of the structures of [H2Fe(CO)4] (261) and [Fe(CO)4(AuPPh3)2] (51), which are illustrated in Fig. 18. 

As an illustration of the phosphorus-carbon analogy, consider the result of replacing a carbon fragment (CR2) by a phosphorus moiety (PR) in each of the common low-coordinate organic compounds (1C-4C) shown in Fig. 1. For example, the replacement of a single carbon in an alkene (1C) results in a phos-phaalkene (IP). Similarly, the phosphaalkynes (2P) are isolobal relatives of the alkynes (2C), and terminal phosphinidene complexes (3P) can be related to the... 

We noted above that the different orbital occupations in the two isolated fragments for CH2, for [Fe(CO)4], 5-9) has no influence on their capacity to form new bonds and is therefore of little importance for the isolobal analogy. The example above illustrates this point nicely independently of the energetic order of the fragment orbitals and the initial electronic occupation, the occupied and empty MO in the two dimers have analogous symmetry properties (5-15). 

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