Isolability, direct linkage to valence

The isolability concept utilized to explain cluster chemistry may simply be regarded as being synonymous to valence. By assigning a valence to organometallic fragments, many structures of organometaalic compounds can readily be explained. The valence of a fragment is deduced from the octet rule or the 18 electron rule. This paper briefly reflects and reviews the isolability concept and points out the its direct linkage to the valence concept. This approach will greatly improve and simplify the teaching of molecular cluster systems at both undergraduate and postgraduate levels. Ample examples of molecules to illustrate the application of valence concept to explain molecular geometry are given.

Let us consider the following simple reactions Li + Br ( ½ Br 2 )  LiBr 2Li + CH 3 Br  LiBr + CH 3 Li 2Na + 2Mn(CO) 5 [Mn 2 (CO) 10 ]  2NaMn(CO) 5 The chemical species Li, Na, Br, CH 3 , Mn(CO) 5 can be regarded as being monovalent. That is, having one available valence electron to participate in a chemical reaction. Hence, these species are isolobal. This can be symbolized as follows: Just like Br, the CH 3 fragment has 7 valence electrons ( 4 from C and 3 from the 3 H atoms). The methyl fragment while it has one unpaired electron, it is short of 1 electron to attain the octet rule. Similarly, the (CO) 5 Mn fragment has 17 electrons ( 10 from the five COs and 7 from Mn. It is short of 1 to attain the 18 electron rule. Thus, the (CO) 5 Mn fragment can be considered to carry 1 unpaired electron in its participation on bond formation to form simple or complex molecules. Similar fragments can be considered as being monovalent as is the case with Group 1 elements (alkali metals ) or Group 7 elements ( halogens ). A removal of one hydrogen atom from the CH 3 fragment produces H 2 C: radical. It has a total of 6 electrons and can be regarded as a divalent species. In this way it has a resemblance to Group 2 elements ( alkaline earth elements) or Group 6. The metal carbonyl fragment (CO) 4 Fe has 16 electrons ( 8 from the 4 COs + 8 from Fe). It is short of 2 electrons to attain the 18 electron rule. Just like an oxygen atom, it can be regarded as a divalent fragment, [ (CO) 4 Fe: ] with two electrons to share and form compounds. Removal of H atom from the CH 2 radical generates the CH fragment which may be regarded as being trivalent . Likewise, the fragment (OC) 3 Ir can be regarded being trivalent .The carbon itself will be considered to be quadrivalent. This is illustrated in Table1.The classification of selected fragments into valence types is also given in Table 2. The chemical species which have the same valence (isovalent) are isolobal.

Interaction of fragments to form molecules and clusters
The formation of simple molecules and clusters can simply be viewed as involving the interactions of appropriate fragments. This is illustrated by the following selected examples given in Figs 1 to 6 .

Monovalent fragments
The interaction of monovalent fragments to form molecules are illustrated by M(CO) 4 ( M= Co, Rh, Ir) and M(CO) 5 (M=Mn, Tc, Re) in Fig.1. below.

Divalent fragments
Interaction of two divalent fragments ere expected to produce a double bonded molecule. This is illustrated in Fig. 2.

Tetravalent fragments
The valence concept is readily applied to clusters involving tetravalent fragments as well. For instance the formation of Fe 5 C(CO) 15 complex can be considered to arise from the interaction of C (V=4)with 5Fe(CO) 3 5 fragments. Finally, the complex C p (CO) 2 FeSnCl 3 can be viewed as being formed from the fragments C p (CO) 2 Fe(V=1), Sn(V=4) and Cl(V=1). This is summarized in Fig. 5 and more examples are given in Fig. 6.

Conclusion
The relatively recently introduced isolobal concept appears to be shrouded in mystery when teaching it at undergraduate level. This mystery vanishes when it is simply regarded as synonymous to valence. In so doing, many geometries of clusters can readily be explained with pleasure.