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We consider the Kondo-Hubbard model with ferromagnetic exchange coupling $%J_{H}$, showing that it is an approximate effective model for late transitionmetal-O linear systems. We study the dependence of the charge and spin gaps$\Delta_{C}$, $\Delta_{S}$, and several spin-spin correlation functions,including the hidden order parameter $Z(\pi)$, as functions of $J_{H}/t$ and$U/t$, by numerical diagonalization of finite systems. Except for $Z(\pi)$, allproperties converge slowly to the strong-coupling limit. When $J_{H}/t\sim 2$and $U/t\sim 7$ (the effective parameters that we obtain for Y_2BaNiO_5),$\Delta_{S}$ is roughly half of the value expected from a strong-couplingexpansion.Comment: 6 pages in RevTeX, 8 ps figures. Accepted to be published in Phys.  Rev.

Influence of finite Hund rules and charge transfer on properties of Haldane systems