20 INORGANIC CHEMISTRY ESSENTIALS
Structure – activity relationship studies have shown that the complex forms
stable adducts with DNA.^11 Two unsubstituted cyclopentadienyl ligands
appear to be important for activity because the compound (MeCp) 2 TiCl 2 has
been found to be biologically inactive. The corresponding molybdenum -
containing metallocene, Cp 2 MoCl 2 , appears to have a different mode of cyto-
toxic (antitumor) activity because it does not form stable DNA adducts under
the same conditions as the titanocene. Although the molybdocene readily
forms soluble adducts with the amino acid cysteine and physiologically preva-
lent sulfur - containing compounds such as glutathione (see Figure 7.37 ),
thiol derivatives of a molybdocene dichloride have been shown to lack
antitumor activity. Researchers in the fi eld have concluded that the biological
chemistry of each of the metallocene dihalides is unique. The ongoing
study of organometallic metallocenes as antitumor agents have been
reported in various articles as referenced here.^12 The anticancer agent cisplatin,
cis - dichlorodiammineplatinum(II), also obeys the 16 - e rule.
Figure 1.9 Molecules obeying the 16 - e and 18 - e rules.TiClClOC COOC COCOCOOC COOCFe Fe 2Fe 1COCO
COCOC
OOCCOCO
OCOCOC
OCH 3 N ClH 3 N Cltetracarbonylnickel(0)
Ni(0) = 10 e (4s^2 3d^8 )
4 CO = 8 e
Total = 18 epentacarbonyliron(0)
Fe(0) = 8 e (4s^2 3d^6 )
5 CO = 10 e
Total = 18 eFe 1 (0) = 8 e
3 terminal CO = 6 e
2 bridging CO = 2 e
2 Fe-Fe bonds = 2 e
Total = 18 e for each Febis-η^5 -cyclopentadienyl-
dichlorotitanium(II)
Ti(II) = 2 e
2 cp = 10 e
2 Cl- = 4 e
Total = 16 ecis-dichlorodiammine-
platinum(II)
Pt(II) = 8 e
2 NH 3 = 4 e
2 Cl- = 4 e
Total = 16 e+ferrocenium ion
Fe(I) = 7 e
2 cp = 10 e
+ charge = -1 e
Total = 16 eNi FePt Fe