The series of terminal zirconium chalcogenido complexes (eta5-C5Me4R)2Zr(E)(NC5H5)(E = O, S, Se, Te; R = Me, Et) has been synThesized by the reactions of (eta5-C5Me4R)2Zr(CO)2 with either N2O or the elemental chalcogen (E = S, Se, Te) in the presence of NC5H5. The tellurido complex (eta5-C5Me5)2Zr(Te)(NC5H5) reacts instantaneously with N2O to give the oxo derivative (eta5-C5Me5)2Zr(O)(NC5H5). The hydrochalcogenido derivatives (eta5-C5Me5)2-Zr(EH){eta1-OC(Ph)=CH2} have been obtained by the reactions of (eta5-C5Me5)2Zr(E)(NC5H5) with PhC(O)CH3, thus demonstrating the basicity of the chalcogenido ligands in these complexes. The structures of the chalcogenido (eta5-C5Me4Et)2Zr(E)(NC5H5)(E = O, S, Se, Te) and hydrochalcogenido (eta5-C5Me5)2Zr(EH){eta1-OC(Ph)=CH2} (E = O, S, Se) complexes have been determined by X-ray diffraction, thereby allowing a comparison of Zr-E single and Zr=E double bond lengths to be made for a series of structurally-related compounds. A consideration of the difference in lengths of the M-E single and M=E double bonds, with respect to the difference in single and double bond covalent radii of the chalcogens, leads to the conclusion that the double bond covalent radius of zirconium in the [(eta5-C5R5)2Zr] system is ca. 0.07 angstrom shorter than its single bond covalent radius. For the heavier chalcogens (S, Se, and Te), the Zr-E single and Zr=E double bond lengths may be predicted reasonably well on the basis of the sum of the covalent radii of Zr and E. In contrast, both the Zr-O single and double bond lengths are anomalously short, a result that is consistent with Coulombic stabilization by an ionic contribution, i.e. Zr(delta+)-O(delta-). Thus, the zirconium-oxo interaction in Cp(Et).2-Zr(O)(NC5H5) is proposed to be represented by the resonance structures Zr=O <-> Zr+-O-. In contrast, for the heavier chalcogens, the resonance form Zr+-E- is considered to play a less significant role in describing the bonding within the zirconium-chalcogenido moiety.