The isocarbonyl-ligated metallocene coordination polymers [Cp*M-2(mu-OC)W(Cp)(CO)(mu-CO)](infinity) were synthesized with M = Gd (1, L = THF) and Dy (2, no L). In a zero direct-current field, the dysprosium version 2 was found to be a single-molecule magnet (SMM), with analysis of the dynamic magnetic susceptibility data revealing that the axial metallocene coordination environment leads to a large anisotropy barrier of 557(18) cm(-1) and a fast quantum-tunnelling rate of similar to 3.7 ms. Theoretical analysis of two truncated versions of 2, [Cp*Dy-2{(mu-OC)W(Cp)(CO)(2)}(2)](-) (2a), and [Cp*Dy-2(OC)(2)](+) (2b), in which the effects of electron correlation outside the 4f orbital space were studied, revealed that tungsten-to-carbonyl back-donation plays an important role in determining the strength of the competing equatorial field at dysprosium and, hence, the dynamic magnetic properties. The finding that a classical organo-transition-metal bonding scenario can be used as an indirect way of tuning the rate of quantum tunnelling potentially provides an alternative chemical strategy for utilizing the fast magnetic relaxation properties of SMMs.