The (Ph(3)P)(2)Pt complexes (3) of three members of a homologous series of tricyclo[3.3.n.0(3,7)]-alk-3(7)-enes (2a-c) have been prepared by allowing these highly pyramidalized alkenes to displace ethylene from (Ph(3)P)(2)PtC2H4. The X-ray crystal structures of 3a-c show that pyramidalization angles at the carbons bonded to platinum increase from an average of phi = 48.3 degrees in 3c to phi = 55.1 degrees in 3b to 62.3 degrees in 3a. In addition, as predicted computationally, the lengths of the bonds between the carbons attached to platinum increase and the C-Pt bond lengths decrease from 3c to 3a. The H-1, C-13, P-31, and Pt-195 NMR spectra of 3a-c provide information about how pyramidalization at these carbons affects the electronic structures of the complexes. The changes in the C-13, P-31, and Pt-195 chemical shifts and also the changes in the C-13-P-31 and P-31-P-31 coupling constants are consistent with the expected increase in back-bonding from the HOMO of (Ph(3)P)(2)Pt into the empty, pi* LUMO of the alkene as pyramidalization increases from 3e to 3a. The increase in the one-bond C-13-Pt-195 coupling constant from 3c to 3a is found to be linearly related to the decrease in the P-31-Pt-195 coupling constant. This finding indicates that, with increasing pyramidalization, donation from the pi HOMO of the alkene into the 6s orbital of platinum also increases but at the expense of donation into platinum 6s from the in-phase combination of phosphine lone pair orbitals.