= Solution
In a <metallocene>, several orbitals with substantial <metal> $d$ character are <nonbonding molecular orbitals> or only weakly bonding or antibonding with respect to the strong Cp–<metal> framework. Adding or removing a small number of <electrons> from them can therefore change <oxidation state> or <spin> without destroying the principal ring–<metal> bonds. This allows stable compounds on either side of the 18-electron count: for example, the usual neutral counts for <chromocene>, <ferrocene>, <cobaltocene> and <nickelocene> are respectively \b[16, 18, 19 and 20]. The <18-electron rule> remains a useful reference, but occupation of a weakly <antibonding orbital> is much less disruptive than occupation of a strongly <antibonding orbital>. Redox stability also depends on geometry, <substituents> and reaction conditions, so this is a bonding explanation of the observed flexibility rather than a guarantee of equal stability for all counts.
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