The coordination strength can be
assigned to the additional bonding resulted from the p-acceptor
character of pyridine groups (see e.g. reference [1b] vol. I, p. 22,
p. 32). The averaged configuration in the frame of D2d symmetry
corresponds to a (b1)N/5(e)2N/5(b2)N/5(a1)N/5 molecular configuration,
where N is the number of d electrons. Although the geometry
is no longer rigorously octahedral, the procedure allows the calculation
of average 10Dq parameters.