Coordination compounds square planar fields

Although Chapter 25 does not address directly why some compounds with coordination 4 are tetrahedral and some are square planar, it is possible to surmise that the answer lies with (1) Crystal Field Theory and the energies of the d orbitals involved bonding and (2) how many unpaired electrons the metal complex has. 

Some square-planar compounds of the type [Ni(AA)2]X2 are reported to undergo reversible thermochroic behavior when heated beyond the temperature range for dehydration. This behavior has been attributed to reversible square-planar to octahedral conversion.87 88 However, the thermochroic behavior of the similar copper compound, [Cu(AA)2](C104)2 (where AA is N,N-di-ethylethylenediamine), does not involve a change in coordination number rather, a change in the in-plane field strength appears to be the cause.89... 

Note that a combination of various types of potentials can be activated. For example, the coordination geometry for octahedral transition metal compounds can be modeled by 1,3-nonbonded interactions in combination with a multiple harmonic function. This is the approach used in the MOMEC97 force field for a number of metal complexes. Also, for four-coordinate compounds one or both of these potentials can be combined with a plane twist potential that enforces square-planar geometry. 

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