Reaction of the ynamine MeC=CNEt2 with the pentatetraenylidene complexes [(CO)(5)-M=C=C=C=C=C(NMe2)(2)] (M = Cr (3a), W (3b)) at room temperature affords the new heptapentaenylidene complexes [(CO)(5)M=C=C=C=C=C(NEt2)C(Me)=C(NMe2)(2)] (M = Cr (5a), W (5b)) in good yield. The spectroscopic data and the X-ray structural analysis of 5a indicate that these amino-substituted metallacumulenes are best described as hybrids of a cumulene and several strongly polarized alkynyl structures. For 5a and the related metallacumulenes [(CO)(5)M=C=C=C(NMe2)(2)] (M = Cr (1a), W (1b)), [(CO)(5)Cr=C=C=C(NEt2)C(H)=C(NMe2)(2)] (2a), and 3a,b the molecular first hyperpolarizability beta was determined by hyper-Rayleigh scattering experiments. The insertion of a cumulated Ct unit into a metallacumulene quadruples beta, while the insertion of a conjugated C-2 unit results in only slight enhancement of the second harmonic generation response. Substitution of tungsten for chromium has only a minor effect on beta. All observed beta values are reasonably high compared with those of other organometallic compounds.