  To accelerate exploration of chemical space, it is necessary to identify the
compounds that will provide the most additional information or value. A
large-scale analysis of mononuclear octahedral transition metal complexes
deposited in an experimental database confirms an under-representation of
lower-symmetry complexes. From a set of around 1000 previously studied Fe(II)
complexes, we show that the theoretical space of synthetically accessible
complexes formed from the relatively small number of unique ligands is
significantly (ca. 816k) larger. For the properties of these complexes, we
validate the concept of ligand additivity by inferring heteroleptic properties
from a stoichiometric combination of homoleptic complexes. An improved
interpolation scheme that incorporates information about cis and trans isomer
effects predicts the adiabatic spin-splitting energy to around 2 kcal/mol and
the HOMO level to less than 0.2 eV. We demonstrate a multi-stage strategy to
discover leads from the 816k Fe(II) complexes within a targeted property
region. We carry out a coarse interpolation from homoleptic complexes that we
refine over a subspace of ligands based on the likelihood of generating
complexes with targeted properties. We validate our approach on 9 new binary
and ternary complexes predicted to be in a targeted zone of discovery,
suggesting opportunities for efficient transition metal complex discovery.
