Microwave rotational transitions for J'<--J=2<--1 and 3<--2 were measured in the 7-11 GHz range for three isotopomers of (C5H5)In. Similar transitions (up to J'<--J=5<--4) for eight isotopomers of (C5H5)Tl were measured in the 5-15 GHz range. The rotational constants of the most abundant isotopomers are B(Cp Tl-203)=1467.9730(11) MHz, B((CpTl)-Tl-205)=1465.0723(14) MHz, B((CpIn)-In-113)=1809,9785(30) MHz, and B((CpIn)-In-115)=1800.8199(18) MHz (Cp=C5H5). The quadrupole coupling strengths for the indium compounds are eQq((CpIn)-In-113)=118.397(69) MHz and eQq((CpIn)-In-115)=-119.981(31) MHz. Spectra for single-substitution C-13 isotopomers were seen in natural abundance. Deuterated samples of CpTl were prepared to obtain spectra for deuterium-substituted isotopomers. Analysis of the spectra allowed the determination of the following structural parameters; for (C5H5)Tl, r(Tl-C-5,)=2.413(3) Angstrom, r(C-C)=1.421(10) Angstrom, r(C-H)=1.082(9) Angstrom and angle C-5-H=0.9(2)degrees (C-5, represents the planar, 5-carbon ring of C5H5) and for (C5H5,)In, r(In-C-5)=2.314(4) Angstrom and r(C-C)=1.426(6) Angstrom. A Kraitchman analysis was performed on the available isotopomers for comparison of r(0) and r(s), values. Both structural analyses indicate that the hydrogen atoms in (C5H5)Tl are bent slightly out of the carbon plane away from the coordinated metal atom. (C) 1997 American Institute of Physics.