2's complement of negative fractional number
2's compliment can only be used to represent integers. 

However, one solution, you could use two 2's compliment values, one to scale the other. For example, to represent -5/8 you could have two variables, containing values (-5, -8) or for simpler arithmetic, (-5, -3) where the -3 represents the number of bit shifts.

Another solution could be to have an implicit shift. For example, you could use a 16 bit, 2's compliment number to represent values from -256 to +255. In that case, the most significant bit would be the sign, the 8 lower bits would be the integer, and the lowest 7 bits would be the fraction. 

You could arrange this however you like. You could, for example, just use 8 bits, and have the following bit significance:


(bit 7) Sign
(bit 6) 8's
(bit 5) 4's
(bit 4) 2's
(bit 3) 1's
(bit 2) halves
(bit 1) quarters
(bit 0) 8ths


This would give you values in 8th increments, in the following ranges:

Positive: 0 to 15.875
Negative: -0.125 to -16

As per AndrejaKo's comment, this is a fixed point representation.

An example of possible representations using a 4-bit binary number

Binary | Regular 2's comp. | /16 2's comp    | /8 2's comp
------ | ----------------- | --------------- | ------------

0000     0                   0      (0)        0
0001     1                   0.0625 (1/16)     0.125 (1/8)
0010     2                   0.125  (2/16)     0.25  (2/8)
0011     3                   0.1875 (3/16)     0.325 (3/8)
0100     4                   0.25   (4/16)     0.5   (4/8)
0101     5                   0.3125 (5/16)     0.625 (5/8)
0110     6                   0.325  (6/16)     0.75  (6/8)
0111     7                   0.4375 (7/16)     0.825 (7/8)

1111     -1                  -0.0625 (-1/16)   -0.125 (1/8)
1110     -2                  -0.125  (-2/16)   -0.25  (-2/8)
1101     -3                  -0.1875 (-3/16)   -0.325 (-3/8)
1100     -4                  -0.25   (-4/16)   -0.5   (-4/8)
1011     -5                  -0.3125 (-5/16)   -0.625 (-5/8)
1010     -6                  -0.325  (-6/16)   -0.75  (-6/8)
1001     -7                  -0.4375 (-7/16)   -0.825 (-7/8)
1000     -8                  -0.5    (-8/16)   -1     (-8/8)


As you can see, 1011 represents -5 in two's compliment. So if you have an implicit bit shift of 3, (equivalent to divide by 8), it represents -5/8. 

Similarly, if you are using 5 bits, where you have an implicit divide by 16, the bit value would be 10110, where the leading 1 denotes the sign, and 0110 in the context of the 2's compliment signed bit represents 5/8. 

So here is where your "0110" comes from:

Binary | Reg. 2's comp. | /16 2's comp    
------ | -------------- | --------------- 
11111     -1              -0.0625 (-1/16)
11110     -2              -0.125  (-2/16)
11101     -3              -0.1875 (-3/16)
11100     -4              -0.25   (-4/16)
....
10111     -9              -0.5625 (-9/16)
10110     -10             -0.625  (-10/16) &lt;== where "0110" equals -5/8
10101     -11             -0.6875 (-11/16)