The spectroscopic and structural characteristics and the relative reactivity of several TpMsCu(olefin) (olefin = ethylene, 1, 1-hexene, 2, allyl ethyl ether, aee, 3, cyclohexene, 4, and styrene, 5) complexes bearing the bulky hydrotris(3-mesitylpyrazolyl)borate ligand have been examined. Experimental data, including an unusual high-field chemical shift in the ¹H and ¹³C NMR spectra, and DFT theoretical calculations support the proposal that the copper-olefin linkage is mainly sustained by σ-donation, lacking a substantial degree of π-back-donation.