High Spin complexes are mostly stable compare to the low spin complex to avoid the pairing energy that is needed to form low spin unless the CFSE is very high in low spin situation. In these complexes the overall stability is gained from the bond energy. As we know that CFSE contribute only 5-10% of the total stability of the complex thus they form high spin complex though Mn(II) high spin have zero CFSE.

But in case of Fe there is a probability for spin inversion. then why not in Mn. For Mn always HS complexes are the stable one?

The answer here relates to the dn-configuration which is d5 for Mn(II). Being exactly half-filled the HS d5-orbital arrangement is particularly stable. It should be added however that low spin Mn(II) complexes do exist with CN- for example and other organometallic ligands (Cp-). The other common d5 metal is Fe(III). Here the higher oxidation state of the metal means that a lower ligand field is required to give a low spin state. For instance [Fe(bipy)3]3+ is completely low spin, whereas for [Mn(bipy)3]2+ this complex is high spin.

I agreed with David. Moreover spin crossover depends on selecting suitable ligand. It is not completely dependent on metal.

The HS/LS of any transition metal ion depends upon four most contributing factors- the transition series (I, II, III) to which the metal ion belong, the oxidation state of the metal ion, nature of the ligand( crystal field strength) and stereochemistry of complex in addition to SSCO, presence of pi- acid ligands, JahnTeller Effect(JTE) and MAY even on CTTL/CTTM and STERIC factors. So the sweeping statement that Mn (II) always form HS, I fear, is contrary to existence of Mn (II) LS complexes reported in literature.

Yes, all Mn (II) TET but its OCT complexes with WEEK ligands are expected to be HS.

Still two more points which I will, humbly, bring to your kind perusal are:

[I] First about CFSE:

[a] CFSE LARGELY contributes to the HS/LS states(certainly much more than 4-6% ). I site two very well known relations as:

∆TET is appxo. equal to 4/9 of ∆OCT

[ keeping the other factors like the metal ion , the ligand same]

Almost double the CFSE in OCT complexes make them LS and HS in TET complexes.

[b] Still an other point lending credence to the importance of CFSE lays the fact that II and III transition series metal ions complexes even with WEEK ligands are found to be LS because the CFSE is of a congener of II transition series metal is 1.5 times of its counter part of I series. It becomes all the more pronouced in III transition series whose congeners have CFSE 1.5 times more than those of EVEN the II transition series ions.

[II] Now about JTE:

Again, JT E comes into play in many ways. But GENERALLY, it INITIATES from the UNEVENNESS of number of electrons in t2g eg sets.Without going in detail (pausity of space), it is maximum in Co (II) and PROBABELY the maximum in Cu (II) complexes.

THERE IS HARDLY ANY POSSIBILITY OF PRESENCE OF JTE IN HS Mn (II).

Lastly, about the stability of Mn (II) HS. Given a choice, ISOLATED GASEOUS Mn (II) ion should always be present in HS state. Because of two reasons-firstly, symmetrical half filled d^5 (t2g^3eg^2) configuration will bring about stability. Secondly, its EXCHANGE ENERY (calculation method given in literature) is maximum.

I completely agree with the last answer and that of David, the high exchange electronic energy of half filled d5 orbital make it resistance the pairing of its electrons and that will made him need for very powerful ligand to cross to LS complexes