Trend in the infrared stretching of molecular hydrogen to group 8 transitions metals?

While going through old notes, in relation to the [Dewar-Chatt-Duncanson model](https://en.wikipedia.org/wiki/Dewar%E2%80%93Chatt%E2%80%93Duncanson_model), it states that the stretching frequency for $\ce{N2}$ to the group 8 transition metals ($\nu(\ce{NN})$) proceeds as:


\begin{array}{cc}
\hline
\text{Compound} & \nu,~\pu{cm-1} \\
\hline
\ce{[(SiP3)Fe(N2)]-} & 1920 \\
\ce{[(SiP3)Ru(N2)]-} & 1960 \\
\ce{[(SiP3)Os(N2)]-} & 1931 \\
\hline
\end{array}


However, the notes fail to elaborate further.


I would have perhaps assumed that the trend was due to increasing size of the $\ce{3d -> 4d -> 5d}$ orbitals and thus increased pi-donation causing a weaker bond and thus lower frequency, however, this does not fit the trend observed. Thus leads me to my question: how does the Dewar-Chatt-Duncanson model relate to the observed trend?


