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Soft legends tend to co-ordinate to metal ions which are already associated with soft legends and inversely hared legends tend to combine those metal ions which already contain hared legends. This phenomenon is termed as symbiosis. However,this he hardness or softness of an acidic or basic venture is not an inherent property of that particular atom but can be changed by the subsistent atoms. The addition of soft, polarizable legends to a hard metal atom can make it a softer acid. The presence of electron-withdrawing substiuent can increase the hardness of metal atom. For example. Attachment of hare base (F-) to B3+ ion makes BF3 a hared acid whereas attachment of soft base (H-)* make BH3 a soft acid. The hare BF3 molecule prefers to combine with hare base (F-) to form BF4- while soft acid BH3 preferably combines with soft base (H-) to give BH4-.
                                           BF          +            F-             →       BF4-
                                       (hard acid)         (hard base)
                                           BH3           +           H-             →      BH4-
                                        (soft acid)         (soft base)
Ina competitive reaction, the reaction
                                          BF3H-        +           BH3F-    →        BF4-            +           BH4-
                                       (hard-soft)        (soft-hard)             (hard-hard)           (soft-soft)

always proceeds to the right.

This phenomenon of symbiosis helps us to explain the preference for symmetrical substitution than to have unsymmetrical substituted products.

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