Twice as reactive: The coordination chemistry of phosphane-functionalized Zr and Hf cycloheptatrienyl-cyclopentadienyl complexes gives rise to unusual secondary interactions associated with the presence of Lewis acidic 16-electron sandwich moieties. These structures can develop weak dative bonds as exemplified by the noncovalent Pd-->Zr interaction in the heterobimetallic {Zr(2)Pd} complex (see picture). Phosphane-functionalized cycloheptatrienyl-cyclopentadienyl Group 4 metal complexes of the type [(eta(7)-C(7)H(7))M(eta(5)-C(5)H(4)PR(2))] (M=Ti (9); M=Zr (10); M=Hf (11); R=Ph (a); R=iPr (b)) have been prepared by the reduction of [(eta(5)-C(5)H(4)PR(2))TiCl(3)] or [(eta(5)-C(5)H(4)PR(2))(2)MCl(2)] (M=Zr, Hf) with magnesium in the presence of cycloheptatriene (C(7)H(8)). In the solid state, the Ti complex 9 a and the complex 11 b are monomeric, whereas 10 a, 10 b, and 11 a form dimers, in which the sandwich units are linked by long Zr-P or Hf-P bonds. Density-functional theory (DFT) calculations indicate that the metal-phosphane interaction is weak, and accordingly, both the Ti complex 9 b and the Zr complex 10 b act as monodentate phosphane ligands upon reaction with [{(cod)RhCl}(2)] (cod=eta(4)-1,5-cyclooctadiene). The X-ray crystal structures of [(cod)Rh(9 b)Cl] (12) and [(cod)Rh(10 b)Cl] (13) reveal that only the latter exhibits a secondary intramolecular Cl-metal interaction. The complex [(10 b)(2)Pd] (14) was obtained by reaction of [(eta(5)-C(5)H(5))Pd(eta(3)-C(3)H(5))] with two equivalents of 10 b. The solid-state structure of 14 reveals a T-shaped Pd(0) center with a long Pd-Zr bond of 2.9709(3) A, which can be interpreted as a weak noncovalent Pd(0)-->Zr(+IV) bond, as indicated by the calculated relaxed force constant of 5.68 N m(-1).